There isn't a cap on cast rate as far as I know. Testing out attack speed of .88 attack time:

Attack time decrease by a 1/x fashion so attacks per second (aka 1/attack time) rises linearly with IAS. So it really isn't a diminishing return. The IAS (and I assume cast speed also) is close to the formula: Attacks Per Second*(1+%IAS)
eg 30 IAS on that .88 attack time weapon is about
1.136*1.30 = 1.477
Maybe this works different with different base weapons though (faster or slower). I don't really care to test it out
Oh yeah this also makes sense because if you had +100% cast rate/IAS, you'd expect to do two times the attack you can every second
This post was edited by ORLYYARLY on Oct 7 2012 01:55pm