That's a good question, and I understand your concern. As you know, the 510 and 520 use different SSD controllers (Marvell 9174 and LSI/SandForce 2281 respectively) and also use different size NAND die chips (34nm and 25nm respectively), and perhaps most significantly, different firmware. The NAND difference should not affect anything, and in theory the RAID software and Windows file system should just see them as storage devices, with their internal operations isolated from each other.
The usual RAID caveats of using different size drives applies of course. If they are not the same size, the total capacity of a pair will be twice that of the smallest size drive. If you combine a 64GB and 128 GB SSD in RAID 0, the volume size will be 128GB. The rest becomes inaccessible spare area. Any performance differences will cause the faster drive to finish before the slower one, so the slower one sets the performance limit. But don't worry much about the 510 being slower, in some situation it can be faster than a 520.
Frankly, the only way you can really tell if it will work is to try it. Create the RAID 0 volume out of them, but don't immediately make it your OS drive. Play with it as you said. Run benchmarks on it, copy files to it, like pictures, and click you way through them quickly to work out the volume. Play with it for up to a week, or for at least a few days before committing important data to it.
Not many people have done what you are considering, but a few I have heard of have, and did not say it was a disaster. I should try that myself. Let us know how it works for you.
