If you want to keep the warranty, go with nismo springs. Springs will give you some added performance (lower center of gravity) but by way of actuall grip, it helps but its only ok IMO. coilovers will give you the aggressive lowered look and give you a great jump in handling. you can adjust the shock/strut to the stiffness and height you prefer on most applications vs springs that give you .7 or 1inches drop etc... personally if your trying to stay cheap right now and not have a "full fledge" race car, i'd do sway bars. Stillen has them at a great price:
Stillen Sway Bar 07+ G35 Sedan/ G37/ Nissan 370Z
or you can go with hotchkis but they're a little more. also when getting springs you have to consider wheels sizes and offsets you wants since you cant adjust springs.
GL I hope this helps a little with your situation.