Does powder coating wheels weaken them in any way?
 

Because of the baking process? I know anytime you add heat to something, you risk its structural integrity weakening a little. Does that apply to wheels?

Yes. OEM cast wheels can weaken and crack if exposed to high heat. Nissan issued a TSB when I worked there telling customers that powdercoating or chroming the OEM wheels would void the warranty due to increased risk of cracking. I have personally seen 5 350Z rims with cracked spokes that had been powdercoated.

If the wheels are knock offs and not forged, then you might have problems with them losing their durability during the powder coating process.