Pening, do you have any examples on how you configured gpus of the different brands from the same series? I've looked around where you pointed but all I find are configs of 7970 and 7950 and different series
There hasnt been a deterioration in speed, always been that way, about 100 Kh/s below what wiki mining says you can get. under SHA256 i had the same issue. i've put it down to the PC hardware because had the same with the another card, but i've also dont really know what im doing with the settings.

for example everyone seems to set the intensity to 19 or 20, but above 13 i spaff HW errors.
the best tutorial is one by the author of cgminer, on this forum somewhere. but the readme docs are very good too, and iirc theres some examples for two cards. cgminer will simply recognise both cards and use them, if you want to tune their settings individually you set the values seperated by ",".