'AMD Overdrive'
 

I have a 6950 , is this feature in ccc the same as overcloking to a 6970? apart from the extra shaders?

More or less. I don't know what the stock speeds of a 6970 are, so I don't know if the 6950 will go that high without bios changes or not though.

I run both my 6950's with the shaders unlocked (modded 6950 bios), but no overclock or voltage changes. That's more due to heat concerns than anything else though - crossfire on these cards does bad things for the airflow and they get hot enough without me pushing them any harder.

Does unlocking shaders give a performance hit?

S!

Here the differences between 6950HD and 6970HD. Flashing the BIOS of a 6950HD can make it a 6970HD but solely at your own risk. OverDrive is nothing but an overclocking utility. You can adjust the core/mem and also Power ie. go past the TDP value of 300W.

AMD Radeon™ HD 6970 GPU Feature Summary

Up to 880MHz Engine Clock
2GB GDDR5 Memory
1375MHz Memory Clock (5.5 Gbps GDDR5)
176 GB/s memory bandwidth (maximum)
2.7 TFLOPs Single Precision compute power
683 GFLOPs Double Precision compute power
TeraScale 3 Unified Processing Architecture
1536 Stream Processors
96 Texture Units
128 Z/Stencil ROP Units
32 Color ROP Units
Dual geometry and dual rendering engines


AMD Radeon™ HD 6950 GPU Feature Summary

Up to 800MHz Engine Clock
2GB GDDR5 Memory
1250MHz Memory Clock (5.0 Gbps GDDR5)
160 GB/s memory bandwidth (maximum)
2.25 TFLOPs Single Precision compute power
562.5 GFLOPs Double Precision compute power
Double slot form factor
TeraScale 3 Unified Processing Architecture
1408 Stream Processors
88 Texture Units
128 Z/Stencil ROP Units
32 Color ROP Units

No, it gives a boost.

The 6970 bios flash has the effect of unlocking the additional shaders, overclocking the GPU and VRAM, and boosting the voltage. It's that last part that can potentially damage your card (some can handle it, some can't - you can't know until you've found out the hard way).

The bios I use is my own stock bios, but with a single line of hexadecimal modified to unlock the additional shaders that have been artificially disabled. There's also a few modded version out there that unlock the overclock limits too, so you can go higher than the normal 6950 cap, but without being forced to increase the voltage if you don't want to (though lower voltage means less overclocking ability obviously).