Don't bother flashing a card with 2.2 ns RAM with a 5950 BIOS... it's useless... the RAM won't go much higher than 950 no matter what you do, and it's easy to overclock it to near that without using a different BIOS.
On the other hand, if you have 2.8 ns RAM, the extra voltage provided does help with overclocking and you can get the RAM up around 950.
You'll get more out of the GPU by flashing, but in my attempt at it, it doesn't make a whole lot of difference in performance. I run my 5900 with 2.2ns RAM at 490/940... with the 5950 BIOS I was able to run it at 530/950... hardly any difference in performance, so I decided to put the origional BIOS back and remove some stress from the added voltage the 5950 BIOS provides.