I've seen a similar problem.  I only have 1 usb asic, when running under asteroid or cgminer on osx the pool gets only about 1/2 speed, but if I take the same asic & run it on my PC I get full speed out of it.  Looks to me the problem is with cgminer and number of asics doesn't matter.  I will be trying bfgminer on osx this evening if time allows.

So, I have a usb asic & am currently running it on my pc and I get around 330 MH/s.  When I set it up on my mac mini (which is always on as it's my HTPC rig) with cgminer I only get around 170 MH/s.  Has anyone else seen this?

I would prefer to run it on my mac mini as it's always on & I only boot up my PC for gaming, so it would save me about 80 watts if I could get it running at full speed on my mini.