1st part of the question: Overclock does give some extra performance. However the extra heat it generates isn't worth the gain for most people. You start entering diminishing returns.
1a: Overclock isn't guarantee at any limit or speed. Even though I have i7-4790k I haven't touch the overclock settings at all as 4.0Ghz is good enough for what I need for a long time.. Some i7's can reach 5.0Ghz while some won't even past 4.6Ghz
2nd part of the Question: Yes aftermarket coolers provide better thermal legroom when overclocking.
3. I would disagree just because having the same clock speed does not mean it will give the same performance. The reason is newer generation cpu has different instruction sets and architecture on how it transfer and process data might be slightly different which software is programmed used and optimized for. Not having the instruction set means the CPU has to handle the problem the longer way.
I know a lot of things about it.
The 950 is 30% slower from the i7-4790k at stock speeds or boost speeds. I have to reach nearly 4.55GHz to have almost the same performance as the i7-4770k I think it was.
I was wondering that will the CPU be 100% stable at that core speed and be coolen with that liquid cooling system, "Cooler Master - Neptun 240M".