You still see some difference between 256 and 512....As for the BIOS i wouldnt count that you can unlock it...I have yet to see an 7800GS flashed and working as an 7950GT. One attempt i saw online succeded but still in benchmarks nothing changed. From what i remember the extra pipes just werent there.
I think the difference between 256 and 512 would only really be noticable at higher resolutions, granted the 512 card would have an easier time with higher texture filtering, aa, and such, I don't think the difference at the resolution CDAwall runs would net a very noticable difference. But if it's only a $25 shot for the 512 card over the 256, then it wouldn't hurt, especially if planning on going with a higher res monitor. Granted, it depends on how the rest of the system could cope too and what that card would be capable of in higher resolutions with the current build, and it's own rated performance for current games. I've seen a lot of benches and tested some systems, and I just didn't see a big enough difference if any at resolutions below 16x12 to really justify the extra cost, but it depends on a user's needs, wants and requirements.
I also gotta agree with ya on the 7800GS mods, I've seen some mad OC's, but with a verified SN of pipes unlocked, it seemed they just weren't there. The benchies didn't show much for a difference.
CD, keep us posted, I noticed a very noticable difference in performance when I modded my x850pro from 12pipes to 16 pipes..if you are successful, you will know it!