unless I'm missing something doesn't afterburner have a voltage setting you can change? at least have read that you could change it with the 5850 twin frozr ii(have had 1 for the past several months but not ventured into ocing it with afterburner yet, just recently had a little play with the ccc overdrive to 750mhz clk left the mem at stock).
I've been reading a few reviews of the toxic all have been able to unlock the shaders(tho as you say sapphire may have sent them hand picked ones), I', tempted to order 2 for xfire so hope both unlock.
reviews for the different 6950 models(1gb and 2gb) so you can decide which to go for http://www.techpowerup.com/reviewdb/Graphics-Cards/ATI/HD-6950/
ya I wouldnt hold your breath with a unlock - over at newegg.com in the states it gets mixed results for a unlock (just from a small number or reviews).
but still prob be a good cooler if you were going to xfire considering it expires heat out the back