I reckon the 5870 will downclock itself because isn't crossfire basically a replication of graphics cards to boost power. The 5870 will try to replicate the 5970 so it will change its setting accordingly?
Not all the time, I remember some guy SLI'ing 2 7800GT's and his ASUS one (OC'd) kept its clock, his Zotac one (Stock) overclocked itself.
Either way with a good PSU you can OC the 5970 just as much if not more than the 5870.
I thought SLI was different than Crossfire. If my info is correct SLI dose not downclock the faster of the two cards, this was patched back in the 17#.## drivers? And Crossfire has allways downclocked the faster card. Dont know if my info is old, havent heard anything about this in awile.