As an owner of an ATS with
As an owner of an ATS with the 2.5, I'm surprised to hear the IS 250 feels faster, or more fun.
I have beaten IS250's stop light to stop light several times, and the ATS is a blast to drive, handles like a quality sports car. I would put my ATS 2.5 against an IS250 in any performance test or race, autocross or straight line bursts.