I have like 380kh-400kh with a 7950 at 900 mhz on core, i think is normal if you have 400k/h and up.
It would be because N-factor which lowers the gpu hash power and gives a bit to cpu.
However the OP said gpu mining will be always more profitable vs cpu.

I have the same amount of hash on my 7950's and I have 8gigs of ram on the mobo. I could never get my shizz as high as others got and i'v messed with the settings to try and optimize and just gave up