ummm if am not mistaken the stock si rims are alloy. i heard from a number of friends that wanted to powdercoat their alloy rims, not to. cause teh alloy rims are tempered from the factory and when the powdercoater bakes the rims it un-tempers them making them weaker or more prone to cracking. i dont know from firsthand experence, but i heard a lot of ppl saying that.
if the rims are forged just forget what i said. there is no problems with powdercoating forged rims.
The OEM wheels are not "tempered" (wheels are not tempered) or forged... they are low pressure cast wheels... the low heat (300-400 degrees) from powder coating will nto damage them in any way. My PCer told me he is hesitant to PC forged wheels due to the heat... but I have seen people PC even forged wheels with no problems.