BGTV8

So the theory goes like this:

1. Tune the driver - when you cannot go any faster with an OEM setup, move to step 2, but to start, you only need the usual safety and reliability stuff: must have an oil cooler, pads and brake fluids - if you don;t have the sport model - purchase some Akebono's - they are good enough until you get really serious
2. Bars first, our cars need a BIG front bar, leave the rear bar alone, replacement front bar needs some adjust-ability, with stickier tyres - if going r-specs at this stage, you also require 2nd set of wheels and a small trailer to tow it all, so fit one of the hitches off the forum as well. Time to add a decent diff at this stage, I have a KAAZ 2-way with 4.08:1 gears as well as 3.7:1 with a Quaife. The KAAZ is brutal, the Quaife is softer, but there is a time and place for each - it is circuit dependent. I use the KAAZ for those circuits that are stop/go and the Quaife for circuits with a more flowing nature. Since I do not know US circuits, I can't offer any opinion.
3. Next is suspension - shocks and springs - the best you can get for your budget, could be as simple as BC ER thru KW V3 all the way up to JRZ or Ohlins gold-plated goodness. Chuck the OEM suspension bushes and replace with SPL kit, front upper control arms and lower control arm bushes are a must to get the best out of sticky tyres and a rear upright bushing kit is also mandatory - OEM bushes turn to sludge at the first sign of a real workout and that makes the car "sloppy" when you need it taut. At this stage you also need the yaw sensor fix so you can trail-brake at will - not fixing the yaw sensor will destroy your enjoyment of the car. If you are here and do not have a diff in the car, you need to add it now, and you will require r-spec tyres to maximise the mechanical grip available with a stiffer suspension
4. Then fit a race seat and 5 or 6 point harness
5. Now you add power, and go all out to reduce weight - the usual stuff is to remove passenger seat but you can go all out and completely strip the car, but it will affect resale values no matter how careful you are. If staying NA, LTH and a good free-flowing system plus CAI and a tune (maybe clean up the lower inlet manifold as well). If going FI, you need to max out the rear rubber and you will need to think seriously about front track to maintain some sort of handling balance. I have an NA setup with 9.5 x 18 on 265/35R18 or 275/35R18 tyres at ET20 on the front and ET12 on the rear and find this to be well balanced

I reckon on around 1 sec of lap time for each additional 50-70 hp atw, so going from 300 to 500 hp will give you 3-4 seconds of lap time, but it could be the most expensive lap time reduction because it could well cost you an engine - so beware FI and TT especially as it puts a lot of heat low-down into the engine bay. SC is a little better in that it only generates combustion heat, but the difference is marginal.

This is a lot about personal opinion but I have done this a half-dozen times and this approach has worked for me, so it might work for you as well.