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This is a rule of thumb for our cars, in terms of peak hp vs. 1/4 mile improvement, non turbo, at our weight, making our power levels, every tenth of a second is about 5 hp or so (that is a VERY rough estimate!). Of course, WHERE you gain hp is just as important. You could gain no peak hp and still see a 1/4 mile improvement.
First, I think the essential NT bolt on power mods you should have are a GS-T muffler, a CAI, and an underdrive pulley.
The short ram intake is helping you out, but a cold air setup is more effective. A short ram is good (for a 1/4 mile improvement) of about 0.15 seconds or so, a CAI should be a quarter to three tenths (at least that's what I saw for an improvement). CAI is the best NT mod in my opinion.
A gs-t muffler will give you enough air flow out through the exhaust without sounding too loud. I think (in terms of improved 1/4 mile performance, if you drag race) it should give you a tenth or so (I can't be sure, I've never raced without the muffler). It is defenitely worth it, and it's cheap and looks good too  one of the essentials for our cars.
In my experience (and many other people's experience) a header will not improve your 1/4 mile times and won't add much (if any) peak horsepower, BUT that doesn't mean it is a bad mod becasue the header will improve your low end to mid range power. A header seems to work best at around 3000-3500 rpms, which is too low of an improvement for drag racing but is good for street driving. Best bet is to find the least expensive one you can which is painted or ceramic coated.
A high flow cat will add maybe 2 hp, that's it, not really worth it AT THIS POINT. If you have a turbo or lots of NT mods (I mean lots!) then it is far more effective, especially for turbo owners. If you don't care about check engine lights, emissions, or fix it tickets from cops, go for a test pipe. It will be cheaper and it should give more of an improvement than any cat (just keep the pipe diameter the same as the rest of the exhaust). A test pipe might knock off a tenth, maybe. As The1bill would say, exhaust (except for the muffler) isn't that bad, it's the intake needs the help.
An underdrive pulley adds a little bit of power everywhere. The difference in performance feels about the same as the difference between driving with the air conditioner on vs. off. You will notice the improvement everywhere in the rpm rage, but it will be most noticeable in the low end (since there is so little low end to begin with). You can cruise in 5th gear at a lower rpm after teh UDP, and it should take about a tenth off your 1/4 mile time (a great mod for $160, if you buy an AFX pulley).
Hope that helps for the bolt ons.
"Tutto fa brodo."
Todd Scungio
98 RS
15.173 @ 90.70 MPH
And also a 2011 Ralliart
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