What miner are you using to get 458 MH/s at only 985MHz? It takes me like 1010MHz to hit that number with the phatk 3% mod.
I use phoenix with phatk 2.2, I think its mostly due to using a headless Linux setup. I have a Windows machine with a 5870 that only gets 445 @ 985 though its my daily workstation and has Aero and other crap enabled.