You should'nt have a problem getting 500whp out of it...especially if you take your time, and do it right. However, the tune is really important, get a great tune, and you should be set. Myself, really picky. I like high end power, dont get me wrong, but, I wont sacrifice low end for it, but, what's nice about stangs, stock, is they've always had a really good balance, doesnt really matter what gear you're in Anyways, yeah, 500 is a good target #. Depending on cams and tune, you'll have to decide what kind of hp vs torque you want, depending how you drive the car and its gearing. No doubt if you take the time, you can have it perfect.
For me streetability is #1. I want low maintanence and excellent output, I cant wait till I take the time to get my heads and cams done....uhg....I hate how the car feels stock at 3k rpm, especially in 4th gear, just doenst feel like it's pulling like it should, however they say the biggest improvement in the 99+ 4.6 2V was how it ran at 3k and above. Maybe it's just me...but it feels as though it's pulling no better than at 2k, I dunno, wont matter soon.