Sounds like you're at a crossroad, where you need to decide whether to stay N/A or go F/I......I've always been a big fan of N/A (there's no replacement for displacement). I'll probably eventually hit the ~400hp wall and end up wanting more, but I'd really like to go against the grain and see what these cars can do on motor alone. Besides, as tommyroc said, 500HP or more isn't really meant for a daily driven car on street tires. Its hard enough to avoid the temptation to mess around with a measily ~300HP, and its hard enough to keep the back end in the back too......
It sounds like you've got a good start on mods, and what you have will help whether you're N/A or F/I. If you want to stay N/A, then there's heads & cams. After that its probably just a matter of fine tuning to get the most out of your combo, and maybe adding supporting mods if needed (injectors, fuel system upgrades, etc.) Staying N/A without major engine work, you're probably limited to around 425-450HP at the most....still quite respectable for the street.
If you go F/I, then its just a matter of deciding what type of power adder you want to use, and building up from there. Then your only limit is the stock internals, which are good for ~500HP.
Some other things that you can do in the mean time, that will help you either way are:
1 piece aluminum driveshaft
new clutch
billet flywheel
gears
better upper/lower control arms
better panhard bar or watts link
better shocks/struts if you didn't replace them when you did the springs
good castor/camber plates or GT500 upper strut mounts if you haven't already done them.
None of these things will add HP, but will still make a huge difference in acceleration and getting power to the ground.
Then theres always brake upgrades....which also depend on wheel size.....
and a set of underdrive pullies........
There are LOTS of things that you can still do while you decide....LOL
