Stillen Gt3 Intake, Berk Exhaust - what bolt on is next? (non FI)

centex

So I know there are a bunch of views on this and many threads but I wasnt sure if my situation is unique so here it goes:

As stated I here
First Post: New Proud Parent - Blacked out 370Z (pics included)
I have a Stillen Gt3 Intake and Berk Exhaust - so my question is - what bold on mod can i do next to give me the next uptick in power - (and no I do not want forced induction) reason being is this is my daily driver. I live in Austin Texas and its hot and I am in a LOT of stop and go traffic for my daily commute. So again what can I do that will not effect reliability or fuel economy (well not that much)
-Tune?
-Manifold? (was at cars and coffee in houston and saw someone with a flat looking manifold - def a custom or aftermarket job - forget the name on it)
-Do the two go hand and hand - meaning if I get a new manifold is a tune pretty much required (in my experience yes)
-If so what brands and who to go to - (perhaps a custom dyno tune?) - but on the other hand I guess if I do either of these there goes the warranty - ( i did get a 5 year 100k mile warranty on the car. So if Nissan is anything like chevy if something goes REALLY bad - a tune F's everything.
-Do not really want headers - has Berk exhaust and it sounds PERFECT...so dont want to go louder.

Maybe I just answered my own question - ugh its late ...

Sorry if this a "newb" subject but I am just learning these cars and would very much like your input. Is a tune even worth it ? How much on average would it cost and what sort of power gains given exhaust and intake already...?

Sorry for so many "what abouts" .... want to make sure I cover all concerns.

My real goal is to get to 400 crank hp .... given my current mods how far off do you think I am ? Sorry may be worth another thread but just crossed my mind with what Im trying to do if it is possible.

Thanks guys as always