First performance mod you would do?

ghotnit

Yeah I agree with what everyone has said. Cleaning up the breathing is the best place to start. You already have the exhaust so doing the intake side would be next. You can get a full intake (~$400) or you can do K&N drop-ins with Post-MAF tubes (~$200). A tune is not necessary yet. You can get gains from a tune now but wait until you have cleaned up the breathing and you will get better results.

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'14 Nismo AAM 3" TD / AAM Test pipes / AAM CAI / Lowered on Swift / Synolimit Ported TB, upper and lower manifold /DW 1000cc injectors and 340 pump / ECUtek tuned by Hal @ Dynosty / 331whp/268wtq on E85