1. You are correct, Turbo Boost pushes the multiplier to whatever you set the max multiplier to for a single core use. I think by default it is 26 on your chip, when only a single core is in use. You are wise to cut off Turbo Boost when overclocking, because as you probably realize, as you up the base clock without lowering your multipliers, you can be getting into dangerous territory with frying a core or two possibly. The reason all of your cores are running at the same speed is because you have turned off EIST which is Enhanced Intel Speedstep Technology, which when enabled, allows the CPU's frequency and voltage to be dynamically changed, depending on the workload. With this off, it does not have the ability to lower its voltage/frequency, therefore it is always at the max on all cores. You can think of it as the opposite of Turbo Boost. It's like when your car is in neutral (computer is idle), your foot is still pressing the gas pedal all the way down, regardless (EIST disabled, you're sort of "wasting" energy and creating unnecessary heat when the computer isn't working much or is idle).
It should improve multi-core applications in the matter that it is simply overclocked. For instance, you have the CPU multiplier set to x21. The default setting for when all 8 cores are ripping away at full load is x21 with a 133mhz core speed, which gives you the chip's stock voltage: 133x21 = 2799MHz, aka 2.8GHz.
Really all you have done is increased the base clock to 167MHz, and kept the default multiplier the same. 167x21 = 3507, or 3.5GHz as you stated. Now, let's imagine that Turbo Mode was still enabled. Default value for all cores running is x21, meaning all your cores would be running at 3.5GHz, which is the same as what you are doing right now. The problem comes in when Turbo Boost kicks in. The other values that dictate Turbo Boost overclocking go like this, by default:
4-core: x22 = 167x22 = 3.67GHz
3-core: x22 = 167x22 = 3.67GHz
2-core: x25 = 167x25 = 4.17GHz (tsss!)
1-core: x26 = 167x26 = 4.34GHz (ouch!)
So your default Turbo Boost MAX speed would go from 1 core @ 3.47Ghz to 1 core @ 4.34GHz. That's almost a 1GHz overclock on one of your cores if it engages! Rather, thankfully you were smart enough to cut that off, and now, the max speed any single core can reach is 3.5GHz, which is essentially equivalent to running all of your 8 cores at what would be demanded of a single core using Turbo Boost. In effect, you are never demanding more from any single core in your processor than Turbo Boost mode would in its default configuration. This obviously means that Intel designed and knows any single core can handle doing at least 3.47GHz, and that's why they made the default max multiplier what it is. That is playing it safe. If you try to demand 1GHz more out of a core than Intel defines as generally safe... you are asking for that core (or two, since the 2-core multiplier is just x1 below the single core) to go up in smoke.
In reality, by default design, when Turbo Mode ups the multiplier to x26 when only a single core is being used, it sends the other cores into low C-states, or cuts them off, so it removes the heat that would be normally put off from those cores.
In essence, it is a speed vs. heat trade-off. You disable 7 cores that aren't being used, you remove that amount of voltage and heat from those cores, lowering the CPU's temp as a whole, allowing you to get away with the extra heat created by overclocking that single core. Equalizing the overall CPU temperature.
In your situation, you are not cutting off any cores, in fact, you're running them all at full blast, 24/7. This is going to create with (simple, basic logic), 7x the amount of heat that your CPU would put off by default. (As said, that's just a quick assumption/estimate, real life situation could be different).
2. Actually, unless you are having problems, I would not adjust the voltages. Unless you are experiencing any strange activity, such as sudden reboots, strange error messages, etc... then I would leave it alone. Increased voltage means increased heat, and increased heat means decreased CPU lifespan, and greater chances of frying your CPU. So it could possibly make your CPU life shorter, rather than longer. Of course, since you have EIST turned off, it is using full power at all times.
3. I suggest that you re-enable EIST (all it will do again, is dynamically UNDER-clock your CPU cores when they are not needed), and C-states allow your CPU cores to go into lower-power sleep states. These have some relation or compare to your common ACPI sleep states... you are probably familiar with states like S3 (sleep mode), and S5 (computer power is fully off, only standby power is applied for Wake on LAN, etc).
Either of these modes will allow your computer to save power and reduce heat/workload via some method. If you cut EIST back ON, then your cores will run at 3.5GHz under full load, but drop the power/speed of that core until it is actually needed. That's actually a good thing. It could drastically reduce the temp of your CPU. You will save power, lessen heat, and almost definitely lengthen the life of your processor.
BUT -- do not take everything I say as gospel. This is all from what I know.
I would recommend you do some research yourself... wikipedia would be a great place to start. Look up "ACPI", which will explain the different power modes and subtypes. For instance, S(x) means it's a System-wide power state... D(x) means they are device power states, and C(x)-states are specific to your CPU, hence the name C-state.
Here is some info straight from Intel's website about these topics:
"Enhanced Intel SpeedStep Technology (EIST) allows the system to dynamically adjust processor voltage and core frequency, which can result in decreased average power consumption and decreased average heat production. [...] Combined with existing power saving features, Enhanced Intel SpeedStep Technology can provide an excellent balance between providing power when you need it and conserving it when you don’t."
Here is a great write-up of C-states: http://software.intel.com/en-us/blogs/2008/03/27/update-c-states-c-states-and-even-more-c-states/
So, finally, my only suggestion is to leave Turbo Mode OFF, cut EIST and c-states back ON.
My primary concern for you is your core/CPU temperatures and case temperatures. If you ever end up reading this (hopefully you will, I know it has been a few months since your original post), I am curious -- would you please let me know what your temps are? You can get them in the IDCC. I'm really curious to know what your CPU reports, as well as the ambient and PCH temps. Also, are all of your LED things by the skull (CPU load) green? (besides EIST, which I know is off)
Just checking. Under the "monitoring" tab, you can see all of the voltages and the three different rail guages... are they in the green?
Whew, that was a long response. I hope that helps you out, my friend.