i mean if your trying to go faster than 124 then just grab a chip and that should kill the speed limiter, im sure you still had some untouched rpms left.
if you want more power out of the v6 then i dont mean to scare you off but your heading down a rough path. there are plenty of options but theyre all pretty expensive and will require you to be good at working on cars or have the money for all the installs.
ok first off this is all assuming you want 300+ hp with any of the scenarios im about to tell you you are going to want the following:
-8.8 rear end(off a wrecked gt or new)
-new stronger transmission or beefed up stock tranny(new internals)
-complete set of forged internals(especially if you are running any sort of boost)
-true dual exhaust(unless your running a turbo set up, but youll need custom exhaust for that)
-better fuel injectors and likely better pump
-chip and a good dyno tune
ok now theres more but those are basics. now here is a list of popular options:
-supercharger(kits are available with most of what you need.)
-turbocharger(
tma turbos sells full kits now, may be able to make twin turbo kits, any other turbo kit would have to be custom made by a shop)
-engine build(may or may not reach 300hp im not sure, we're talking heads, intakes, stroker kit, cam, valves)
-nitrous(can be combined with any of the above as well but watch out)
other people chirp in if i f'd up anything or you need to add junk