I don't know a great amount on the situ Frag tbh but I did do a lot of reading before I decided on a Crossfire board about this stuff.
Apparently when shared into two 8x lanes you lose at most 10-15% of the overall raw power that you would if you were using twin 16x. At the end of the day I could not justify spending £130+ on a motherboard with twin 16s when I could get one that shares into 2 x 8 for £30.
For some boring figures? Well, a Crossfired 5770 test bed in a review here scored about 17500. That was with an I7 and all sorts. My machine (940 3gb ram) 5770 Crossfire scored 15500.