First performance mod you would do?

Apollo8642

My personal opinion the oil cooler is a must on this car if you plan on tracking, or autocrossing, or if you live in one of the southern states, other wise you probably can live with out it.

As for daily driving non-racing setup, I really liked my Swift-R lowering springs, if you aren't in the market for coils. The Stillen 3rd Gen cold air intakes are nice too. Once you get done with all your bolt ons I would look at the UpRev tuner.

I run hankooks 275 35 in the front and 305 30 in the rear along with running a tad bit of camber, zero toe in the rear, and that has really helped in lengthening the life of the tires though you do get a little bit of road pull specially on bad roads. I'm at 51k and just about to do my 5th set of tires, though some of those tires did see the track which didn't help their life span.

Another item to look at is your diff bushings. They have a tendency of giving up the ghost early in life sense they are liquid fill. Z1 sells a nice Whiteline bushing kit for this with the Nismo diff cover that has the cooling fins I would recommend if you've had a bushing failure.