Best setup for 15 inch wheels would be a C body knuckle so you can run the thicker C body rotor. It takes work to get it on there but it can be done. I designed up a C body knuckle conversion for my car but I never installed it. The knuckle is extra tall so you need a new upper control arm.

The C body rotor is 1.25 inches thick so the Viper calipers fit without needing pad spacers. With some creative welding+ machine work the Viper calipers will bolt directly to the C body knuckles.

I mocked all of this stuff up a few years back but at the end of the day it was just a lot less work (and money) to swith over to 17 inch rims. The 17 inch rims allowed me to build a very easy 13 inch rotor kit with a ton of caliper choices. Sometimes you just have to take the easy path.