Suspension upgrades are great, seriously. No sense in going fast ONLY in a straight line. You can keep a header (singular, it's a 4cyl. . . It's only got one) and exhaust. Freeing up your exhaust helps on FI. A 3" exhaust is great for FI but pointless for NA applications. However a 2.5" aftermarket exhaust is still better than stock for FI or NA.
There's loads of info out there for damn near any question that you might have. To include, suspension, clutches, flywheels, . . . etc.
Lots of people would say go turbochrager besides supercharger. Again, doing the SC thing is my personal preference. It appeals to me more than a turbocharger.
try:
B15sentra.net - Nissan Sentra Forum
My SpecV - The BEST Nissan Sentra SE-R Spec V site in the world
SpecV.net • Portal
BTW, you should honestly get a header and loctite your butterfly screws if not done so already
FAQ:
QR25DE Precat and Butterfly Screw FAQ - MSV