Here is what I have done to my G35 Coupe this year:
1. Added a K&N Exhaust - noticed the performance difference and the sound was nice as well. It took about 2 hours to complete, but after doing it I realize there were many little short cuts I could have taken.
2. Installed a Solo Performane Stainless Exhaust from Zoro in Mississauga Canada. Real nice sound, I did not want something like the kids have today, I was looking for refined sound, they had it. It still has the Coupe rumble but has a Porsche like sound. Gained performance as well, and lots of compliments on the sound.
3. Sprint Booster - OK I was at JRP Auto, the sales guy told me about it and he said "let me put it in and you will certainly buy it". I did, this eliminated the lag from the accelerator, takes a bit of getting used to, but I recommend this mod, big bang for buck ($299 Canadian or $50 us ).
4. UpRev Tune and new Iridium Plugs - They tuned the ECU (94 Octane) and got about 20HP out of it in the track tune. It dyno'd at 243 Wheel HP. I recommend this to anyone. I had them do 3 maps (Track, Factory, Wife (4500RPM limit)).
4. BF Goodrich G-Force TA KDW - Needed this for better traction on the track, I did not like the Pilot's much, had cupping issues.

Next, who knows, I am looking for more traction and HP, maybe 50 more, Let me know what suggestions you have.

Cheers, Colin

 

I've read a few thread about the sprint booster and lots of people are liking it alot

 

What is a motordyne spacer?

 

I've heard good things about the MD spacers, although I don't actually have it...yet other manufacturers also makes spacers but for some reason, these other manufacturers' spacers doesn't provide as much hp improvements as MD.

http://www.motordyneengineering.com/G35.php

I'm curious on the solo-performance exhaust though, what to de look/sound like? any videos of it?