Mods and Warranty
I have a 2001 Black SSEi and I purchased an extended warranty through Warranty Gold for 150,000 miles or 5 years (whichever occurs first). I talked with their reps at length about modding before I purchased the warranty and they are really cool. Basically, as long as ANY MOD you do DOES NOT CAUSE THE FAILURE you take the car in for, then you will be fine.
For example, if your window motor burned out, they can't and won't deny their responsibility to fix it and try and void your warranty if you have an intake or a CAI because that can't be traced to having caused the window malfunction. Now obviously if you do a CAI it'* noticeable and you can switch it back (unless you have a pipe welded for a true CAI in which case it is actually in your fender well) but the CAI shouldn't cause any problems so it won't matter. As far as the pulley, that will generate more HP and torque depending on the size you get. The 3.4 looks just like stock though so most places wouldn't even notice unless you had a failure with that part.
I will tell you, I'm doing a custom cat-back exhaust, high-flo cat, headers, tranny cooler, PCM custom chip, CAI, 180 T-Stat, and modular pulley system (haven't decided on the rockers or cam, etc.). With the above mentioned mods I will have NO problem with warranty issues unless they cause a failure which I don't expect will happen. No part of your exhuast is covered in ANY warranty any way so doing headers, cat-back, and using a high-flo cat will do nothing to the warranty anyway because they don't cover any part of the exhaust system. The pulley looks stock, the t-stat looks stock, and the CAI obviously will not but that isn't going to void your warranty.
Sorry for the story book, but I hope this helps. Bottom line, they have to PROVE your MOD caused the FAILURE before they deny fixing it. And no MOD will cancel the whole warranty anyway.