I have a 2003 g35 sedan and I replaced the pads about 10,000 miles ago. Now my car start to shake when braking at high speeds. I know that it's the rotors. So should I just go resurface them or should I just wait and get the Stoptech Stage 2 Kit?

If you have the cash than get the Stoptechs.

I know that the earlier models have bad brake. So if I resurface the brakes would it be a waste of money?

Yes, the shakes are caused by warped discs. You can either have them
turned or replace them with aftermarket discs.........this your already aware
of. Why not have them turned if they are still within specs? The cause of
warped discs is gererally installing the wheels with an impact wrench........
have them use a torque wrench when tightening your lugs.....quality shops
do this automaticallty! Have the discs turned and use Hawk HP pads but
don't forget to follow the break-in proceedure....no squeeks....80% less
dust and great stopping ability!

The brakes were not bad at all. In fact, the stopping disatnce of the sedan is pretty incredible. People just complained about the pads wearing out too soon.

peaty much
when you resurface the rotors they become thinner and thinner rotors will warp faster and when they will go bad you will have to pay labor again to change them... so its 2xLabor +$$ for resurface
so for me it is a waste of money

I don't have any doubts that the g25 have one of the best braking system out there. There have just been a few complaints about the rotors getting wraped and the pads wear out very quick. But is it wroth getting cross or slotted rotors? I've noticed after installing aftermarket rotors the brakes are not as good anymore.

Quick question, what should the lug nuts be torqued to?

The 05 service manual calls for 80 ft. lbs.