Curb weight for the SRT is 4365 from spec sheet. A Whipple 2.9l SC is about 60 lbs and can add 180whp. That means 4425 lbs and about 600whp. Thats a really nice 7.375 weight/whp.
A well built IS38 GTI will be around 3000lbs at 400whp I which is 7.5 weight/whp.