Yup, the NPI heads are called "non performance improved" for a reason. I have full bolt ons, a blower, and blower cams, and the peak power was still only at 5500ish.
OP, being a current/former owner of a Non-Performance Improved headed car, here's what I would do (to go faster for cheap):
Option A. Gears. Won't add any power, and it really won't make it a whole lot faster, but it will make it feel a whole lot faster. Probably gain .2 and 1.5 mph in the 1/4 with decent traction. Figure on spending $500-$800 for a complete gear install and parts.
Option B. Nitrous. To any nay-sayers, I think you need to go ride in a nitrous car. It is just so much FUN! And cheap! A COMPLETE nitrous kit, with all the bells and whistles (including the stuff to make it safe) is in the area of $750. And you can pretty much make however much power you want. A "safe" amount on an NPI car would probably in the 150 shot range, and result in low-mid 300s rwhp and lower 400s rwtq.
Option C. PI head swap. This involves swapping on PI heads, cams, and intake. The total cost would probably be in the $750 range for a do-it-yourselfer. With your current mods, would probably give you 260-270 rwhp. It also raises the compression up to 10.5:1, which is good for NA power and nitrous, but not for boost.
Option D. PI motor swap. If a blower or turbo is ever in the future of your car, go ahead and swap in an entire PI motor. This way, you get all the PI parts but without the boost-unfriendly bump in compression. Plus you get all of the aftermarket options that PI cars have that NPI cars don't (turbo kits, Kenne Bell, Tork Tech, etc.). Cost would be in the $1000 range all said and done (assuming you did it yourself), and result in 250-260 rwhp with your current mods.
Looking back at all I've done, I'd go with Option D. It will be a BIG improvement over what you have, plus it leaves a LOT more room to grow. And it really isn't that expensive either. I'm literally about to go pick up a complete PI motor with 38k miles on it for $600. That's cheaper than a gear install around here.