The 15x8 is about a pound lighter. The radius of the wheel to the hub is closer. Both assist in reducing the amount of power needed to spin the wheel up, and brake it down. All the usual lighter weight wheel pick ups (increased handling, less stress on the suspension/parts, better ride, and slightly less sharp steering feel at turn in) apply.
There is, however what some may consider limitations, the first being limited tire choice and the second being the wheel location being flush. I think it a good idea to contact rodmoe. He tracks his ST and runs on the same 15x8 Pro Race 1.2 wheels. You can check out his build thread as well. He is always willing to assist another.