Nope they don't.
Benchmarks show that synthetic benchmarks are hardly anything to write home about over P35. The extra PCI-E lanes don't seem to do much either, I read on XS that one LN2 bencher didn't find crossfire any faster but that might be the case because more PCI-E bandwidth doesn't show much improvement unless used at massive resolutions (which stock benchmarks such as 3DMark are not).
X38 can handle DDR3 better by the looks but with the price of DDR at the moment, seems like a null point to make. I think PCI-E lanes is the only benefit for me because I want to invest in a good raid controller card.