The other missing factor is heat. Depending on location heat can be a big problem with a lot of GPUs.
Asics suffer from even more severe heat issues than gpu's. The first BFL labs units were errible for heat and even the block eruptors suffer from heat issues. They run fine but are not running at there top end due to heat concerns
Yep. 10 vs 13 GH/sec...? Quite some difference with ample cooling.