new to 4.6 motors have a few ??

You can't really approach these engines like you would your previous small and big block OHV's. You can add displacement, but only ever so slight. And since the stroke is already quite long (longer rods on a 4.6L 281-modular than a 5.8L 351W) you ideally want to add bore. The problem is, is that the bore spacing with these engines is already so closely spaced, that you're limited to the amount you're able to increase it by.

A 4V head conversion is possible, but it's a lot of work and money. So much in fact, that you're just as well at this point to work with your existing 2V components in order to reach your desired power levels.

Your 2000GT is already running the PI (Performance Improved) heads and a Windsor Block. Your best bet an N/A power from here on out comes from simple bolt-ons and cams. The PI headed 4.6L's respond much better to simple bolt-ons, than the old 5.0L did, since their volumetric efficiency is far more superior. But again, this only will bring you to a point. Once you're hit the 300RWHP mark, you're going to start having to get creative. Custom cams, ported heads and high flow intakes will take you a little further (another 20-30hp), but at this point, your biggest limitation is going to be the lack of displacement.

What really works well on these engines is Forced Induction. Nitrous is fine too, but you really don't want to hit it much harder than 100-125hp shot on the stock bottom end for longevity sake. Any of the major brand superchargers (Procharger, Kenne Bell, Vortech, Paxton, Allen, Roush, Saleen) will get you into the 350-400RWHP mark on a stock GT with conservative tuning and boost. Much beyond the 400RWHP mark though, you're going to want to look at tearing into the short block and beefing up the stock rods and pistons to handle the increased cylinder pressures.

After that...that sky is the limit. You can reach 600 even 700RWHP with a boosted 2V once the fuel and ignition components have been address. But as with anything else, at this level it's going to take money and time.

It's your money, so you'll have to decide how you want to spend it. As the old saying goes. Horsepower costs money....how fast do you want to go?