Many people are pushing 425+whp on stock internals. Much more than that, and you have to start thinking about building the engine. You can get the APS Single turbo kit for about 5k, and that's about the cheapest. Look at 5-7K depending on the kit. Most of the problems people have is in the tuning. The VQ really doesn't like F/I without a good standalone ECU tuning.

That being said, There is a guy on the Z boards (GoingDeep) that has a built Z with 570 rwhp@20psi Greddy TT setup. First time he ran it at the track, he was so ticked because he could only muster about 14 sec. 1/4. Of course, he was on street tires, and I personally watched him spin all the way through 1st, 2nd, and 3rd.... And it is his daily driver. routinely takes cross country trips in it. Here's his sig pic.