I believe it..
the 5950 is just a 5900Ultra with more voltage to the gpu and ram. And the 5900Ultra is just a 5900 with 256mb of ram. So basically, the 5950 is just a 5900non-ultra with higher voltages and more ram, and the ram does nothing for performance. If you do the bios mod, then you are running a stock 5950 with 128mb ram. When you OC in addition to the 5950 mod, you are going higher than 5950 speeds
Anyways, the 5950 is very much faster than the 5900s at all, so its up to you if you wanna do the bios mod or not, I just think its cool to see Geforce FX5950 in the display properties. I also gained 50mhz on the clock and ~120mhz on the memory with the new bios, stable. So, I guess its worth the "risk", and the risk isn;t really there if you backup your bios first and don't do something really stupid.