I run Stillen 5 position adjustable sways w/ Nismo s tune shocks and springs. Sways set to max on the front and 3/4 max on the rear. Also run stock 18's and stock Potenzas. Mostly street, but 1x/month or so on the track. Reduce the rear when tracking.
I really like this setup. Firm, very stable--"on the rails" in the turns, and doesn't rattle my molars when driving to work.
The warranty issue is a non-issue. S tune doesn't void it and NNA needs to prove that non-warranty mods created the problem in the first place. Tough to prove that non-Nismo sways were the problem. Besides, whaddy'a got to lose? I've got 29k on my stock Potenzas and am ready for new tires, anyway. (no major feathering problems).
There are so many suspension mods out there. Figure out what you want to achieve, and go shopping. nissanperformanceparts is a good start. Stillen.com is great if you decide to go that way. Also, there's a guy on ebay selling Hotchkis adjustabiles @ $379 shipping included. Check it out.
Life is short--have fun.