you will see a difference I get anywhere from 5-10 FPS more playing Crysis on max settings (2X AA so not max but meh)
It may void your warranty, you will be able to flash it back and XFX may not be able to see what you did but I read somewhere that XFX is saying they can tell the diff between a stock un tampered card and one that has been flashed and then flashed back(in that case it WILL void your warranty)
However as long as you dont flash it then try to OC the flash ( I mean clock it past the 6970 clock speeds) you should be fine. It has been said that is some extreme circumstances the unlocked 6950 will be slightly underpowered due to having 2x6pin connectors and not a 6pin and 8pin but that should only happen in a benchmark type of situation where the software is built to max out your card. Under normal gaming you should not max out the 2 6pin connectors. But that is the reason you should not OC the card any further because any OC will push it past what the power connectors will support.
I have the XFX card its unlocked and working great. If you look at 6950-6970 reviews you will see the 6970 heats up a bit more and when you unlock your 6950 it will heat up to the same levels that the 6970 does. So make sure you have a well ventilated case. Also you could use something like MSI afterburner to up the fan speeds a bit. In testing the 6970 will get up to 80-90c I tweaked my fan settings and it wont go above 65c now. I dont care about fan noise though so if that is something that bothers you I wouldn't recommend changing fan speeds it can be a little loud. I also believe at 90c stock fan timings will send the fan to high speed making sure your card wont heat up much past 90c.