Tuning cars & 3000GT 1/4 times

Does tuning the cars in-game actually make a difference? I've played around with the tuning and while I'm still mostly in the dark, I know the difference between a camshaft and a shift knob...However, I've only succeeded in making my car slower. Does manual tuning change ANYTHING for the better?

Also, what's the fastest time possible with the 3000GT?
I've gotten 9.11 with 5 perfect shifts and one overrev.(1st to 2nd is about a tenth of a second long, someone can back me up on that- it's CRAZY)

Any suggestions for bringing that time down anymore?

tuning is vital if you wanna do well. i got my 350Z to run 7's & 8's. you gotta see what rpm you land at after shifting at redline. from there start setting your power to arrive a littel before that so that you're always in your best power range, by adjusting your air:fuel ratio. then once you start making good power set your turbo to make full boost later. you don't need your turbo spooling between 1000 and 3000 rpms if you're only in those rpms for one gear.
other than drag racing the dyno doesn't help very much because it only shows the peak power. set your hp and tq to arrive at the rpms you'll need them most depending on which event you're doing. drift and street X you want your power arriving early and tapering off before 6000+ rpm or so because you're not trying to attain top speed.

ditto. tuning really does matter. i've fine tuned my drift car from everything to gear ratios, suspension and downforce, now i'm ranked 45th (last time i checked)in the world drifting online, woot woot.

other than drag racing the dyno doesn't help very much because it only shows the peak power.

When you start the dyno and you see the car revving up dont hit any buttons and let the car get into third gear. After that the power and torque graphs should be generated, and if you run multiple tests it will overlay the previous run with the current for comparison.