Welcome! A K&N filter is a popular choice but will not do much in the performance category by itself. I would recommmend a C&L MAF system (cnlperformance.com) It replaces everything up to the throttle body, including an open air K&N. Check out the prices on ebay though because you can get one way cheaper than the $400 C&l is asking. I have one on my 94 gt and it works great. As far as throttle bodies, I recommend checking ebay again if you are okay with that idea. I got a three month old 65 mm for my gt for $75 compared to $200 retail. They make a slight to decent difference (I would say 5-10 hp at the absolute most, all at mid to high rpm's). It firmed up the gas pedal wayyyy too much for my liking though, I do not know if they all do this though. Just make sure to disconnect your battery before the install so the computer resets itself and you should be fine with no tuning problems.
Looking to increase the horespower on my GT through the use of bolt-on
> accessories. My limited research indicates that a K&N air filter
> would be a good first step.
>
> I have three questions for the group. I have seen references to
> replacing the stock throttle body with a larger diameter throttle body
> in the fuel intake. What would this do in the way of an HP increase?,
> what reasonable cost is involved? and any issues if this is done to
> the car (any change in tuning, timing, etc.).
>
> 2000 Mustang GT (all stock), Black with grey cloth interior. Great > car!
>
> Thanks in advance for any responses to this question.
>
> Chuck