I know the 20 HP with an exhaust for a Camaro was an exmple. If dyno'ed, I doubt you would see that though.
HP and torque are hard to add to any engine. Torque being the hardest.
On my Vette, GM rated it 345 at the crank. After the mods I did to it and dyno'ed, the only way to know what you are making, I am getting 330 RWHP and 340 RWTQ. Not bad for what I did and bolt ons. Estimated engine HP at the crank now is 380.
I pride myself on all the work and 6 speed Vettes naturally make more HP then automatic Vettes.
My engine mods are:
Z06 mufflers, which I really doubt did anything but take off weight. If anything under WOT, they flow better, at WOT only
Replaced the H pipe with an X pipe.
Polished and MAF screen removal, done weeks after dyno testing.
K&N cold air kit.
LS6 intake manifold.
Running a stock tune still.
Sure a rear turbo would be real nice, but would I invest in that, nope!
I was told LS engines are tuned pretty well from GM to maximize power. I have read many people doing more to them, tuning the crap out of them and then start breaking things and making the engine unreliable. A guy at work just broke his LS engine in his 69 Firebird, the first time he took it to the track this year. He is CONSTANTLY tuning it, thus he will be breaking parts along the way. Sure his is making more power then me, but I just laugh and say speed costs, how fast do you want to go!