I have one on my 96 and felt a gain in lowend of the powerband. I do not have any before and after dyno results but the from the results I have seen on the mach 1 forums its seems to make a decent increase in torque. When it comes to a 4.6 I will take any increase in lowend torque I can get.
I had one on my 01 and lemmie tell ya........IT'S A POS!!!!! i lost 5rwhp accross the power band and 1 rwhp at peak. along with an equal amount of RWTQ. i did however gain both HP and TQ from 6500-7000.
Another note the car ran absolutely ****ty with it on. it ran rough and backfired alot. took it off and it ran fine again.
I would save your 75 bucks. then again with your mods it may help but i don't know. in anycase it only take 20 mins to put it on