For what it's worth here was my progression and experience:
My car arrived stock, then almost immediately I went to a smaller pulley and tune. That put down about 600 rwHP. I later added a Gen 2 Whipple, and my fairly conservative tune now puts down about 735-740 rwHP. To be honest, it's hard for me to notice that much difference with the Whipple for 99% of my usage. While the added HP is "cool", 98% of my driving is conservative street cruising and 2% is 2 track days a year with the GT forum. Even on the track, at my mostly novice level, guys with pulley and tunes kick my butt. I can't keep up with some of the track Mustangs which are way less HP. While I don't regret in any way I did the Whipple, b/c I find the idea of HP so cool, it's hard to say it made a big difference in my enjoyment or usage. That being said, I must admit I still like the idea of HP and think a TT would be neat.
Perhaps what I'm saying is do the Whipple if the money is not a concern and you like the idea of it, or do lots of aggressive driving or racing. Otherwise, if you drive like most guys, probably not a big difference and you will be happy with a pulley and tune.
