...and they are all running through Slush's proxy (one for each, different ports) on an older laptop.
Why are you running separate instances of the proxy? You should only be running 1 instance of the proxy for any number of cubes. I have 3 cubes and 1 instance of the proxy and I get >32GHs on all 3 cubes.
Regarding running slush's proxy on RPi. I haven't seen anyone reporting more than 1 cube on an RPi getting full hashing rate. I tried my 3 cubes on RPi and it pegged the cpu 100% and hash rates were < 16GHs. I have not yet tried bfgminer proxy on RPi. I might get around to trying that tomorrow.