I have 10 rigs + other 3 rigs friend of mine, all of them are 4xGPU, 280x (Sapphire, Asus) 7970 (Sapphire), all sorts of cards combined,
but I never had a problem with ver. 3.x, which is most people stating in here.
For tweaking I'm using MSI afterburner, if you have low hashrates and clocks are jumping,
just try to give more volts.
I did not made any other adjustments.
And as well I'm mining with 2-3 threads with CPU on those rigs.
It's running full throttle and nothing is lagging, everything is running stable.
So I can say there is no problem at all with Claymore miner.