I traded my 07 STI which was remapped and larger exhaust, for the ISF. I always wanted the ISF since I saw its spy shots it was therefore a "programmed" progression for me. On the other hand just last weekend a twin of my STI except that this had a larger turbo driven by a friend went at it flooring the ISF from 2nd gear all the way to the speed limiter. The STi was behind me all the way by just about three cars. All I'm saying is it is hard to beat an STi, value for money.
If you love the Sti, you will love the ISF. Many similar charcteristics. I see the STi as the big brother of the STi, the matured STi.