Thats because exhaust, and fuel intake need to be upgraded along side headers...otherwise your creating a bottleneck in pressure...and killing performance. With upgrading HP with bolt ons..you gotta consider several other things, vehicle wieght, gear shifting, tire spin, wind resistence etc...
-weight reduction (many parts upgraded to the light wieght performance version. didn't upgrade hood to Carbon fibre yet though, cause i want it to look stock)
-larger cylinders, to increase fuel injection and combustion. Preludes run 2.2 stock, mine is 3liter
-upgraded fuel injectors
-cold air intake (with forced air intake)
-upgraded the entire transmission
-low profile racing tires
-and a bunch of other things i'm sure i'm forgetting off the top of my head....
she runs about 300hp, but has a LOT faster accelleration then most stock, or racing cars i go up against. Plus its got AMAZING handling...
I do street race with her, so i'm not just talking out my ass lmao
Its still a Honda.
and if I'd hope you'd beat a 5.0, especially with a turbo.