I don't know where you got that from, but your completely wrong. 910 is very possible, some people have even gotten it above 1ghz.
As far as I know, dual GPU cards are harder to overclock and I'm pretty sure that something around 900 MHZ is the limit for 5970s.Tom's did an overclocking session with the LIQUID cooled
Powercolor HD 5870
and it reached 1076 MHz core I think.