You have the PI intake and cams listed in your mods but no PI heads also there is no headers listed so I assume they are stock as well. With the stock rwhp on the early 4.6's being in the 170-180 range I would give another 15 for the intake 20 for the cams 15 for your partial exhaust, and 10-15 for the other small stuff. Your probally around 235 - 245 or so at the tires. I'm not knocking your car saying that it is weak .........the 98 GT I had when I joined here had Full exhaust (long-tubes, hpipe, flowmasters) PI intake, C&L upper elbow, 70 Tb, BBK coldair, 80 mass air, Pulleys, 180 t-stat, and a custom DYNO tune and only made 238 rwhp, and that was with a very aggressive tune. If you have not had your car tuned I highly recommend it, I picked up almost 20 from my tune going from 219 to 238. Not having the better flowing heads and on top of that having the stock restrictive headers is killing what extra air flow you have added by the mods you have already done. Getting the PI heads and a set of Long tubes would do wonders for your GT.