That is way higher than my experience. My problem with load balancing was that the split was very uneven. But now with the new quota method it may be better.
Ah not sure of the % i mine at 16 gh/s and lost 2 gh/s doing load-balance, and the fact the usb miner leds come on and off and stay on for a sec then go back mining. Like they where waiting on something...
I haven't load balanced since my GPU days. It's just too easy to setup multiple instances now with the usb devices now I just have no need to load balance. But when I did it with my GPU's my stales increased, but no where near 20%. Maybe 1 or 2%.