"There seems to be a never-ending supply of every "bolt-on" you can imagine for these cars. They all seem to give you 5-10 rwhp or maybe a little more or less. They cost so damn much money that I started adding it up. It worked out to be an insane amount of money for little horsepower. Why do we do it, and why does the 4.6 not respond to mods very well?"
Well there's the old adage that there's no replacement for displacement...tho forced induction comes close. The problem is not just the displacement but the entire architecture of the engine. These little engines only have 3.5 inch bores, with lousy bore spacing. So boring them out to 4" (like they should have been from the beginning) is impossible. Scrawny little bores shroud the valves and decrease flow. Four valves help, but are costly. Same goes for blowers.
I happen to believe that if GM had have had the stones to back the F bodies and actually build 'em so they didn't nickel and dime you to death, Ford would have had to respond by ditching the modular engine and redesigning the old 302 and/or maybe adding a 351. OHV V8s with 2.02 inch intake valves are just so much easier and cheaper to build than these mod engines.
As it was the stangs were just build so much better than the firebird/camaro. Plus you gotta remember the nineties to present is a retro era. Probably because of the mellinium change. Vulcan/Road Star/Shadow = Harley, Miata = MG, Viper = 427 cobra, and Mustang = well, Mustang! Ford was smart enough to start making the SN 95s resemble the first gen mustangs. Bye bye competition, and bye bye tweakable muscle cars for the common man.
The fact is FoMoCo has done a bangup job of wringing out a great deal of hp out of these little engines. The ford engineers have left very little to the imagination. They have already covered almost all the bases. It's just a shame the guts aren't forged.