Welcome.

I agree with what others have said about budget. The FFR kit is great starting point but there are plenty of areas that need improvement for an ideal experience. If you want to turn the heat up a bit (as you do), then you need to consider a number of upgrades.

For example, even after selling donor car parts I didn't need, I'm already at $30k and that's getting a really good deal on a donor with low miles, and running a stock engine, stock turbo. I still need to buy wheels & tires (~$1500+), some exhaust bits, get it painted, get a dyno tune and an Access Port, get a passenger seat, 6-point harnesses and bits for the MR2 shifter setup.

Yes you can do a strictly by-the-book no-paint build for $15k if you get a great deal on a donor and make some money off parts, but if you want something that has 300+HP, has adequate intake charge cooling, adequate power delivery to the wheels, nice paint, etc, you are going to be more like $25k to $35k (or even much more depending on how crazy you get). Still a total bargain for what you are ending up with performance-wise IMHO but need to be realistic about the cost to do that.

Examples of things you will probably want to spend money on to do a 300+ HP build "right" that aren't part of the kit or donor:
- Intercooling that works for over 225hp.
- Traction (to put 300+ HP down on an 1800lb car, you are going to need to spend plenty of time and money on this) - Limited slip differential, electronic traction control system, wide wheels with very sticky tires
- Better shifter setup (shifter itself, plus cables, linkage and brackets0
- Small bits like Rori's radiator spacers, Mechie's vents
- Custom 3" exhaust Exhaust
- Power upgrades required to hit your 300-400hp power numbers: Turbo, fuel pump, injectors. Possibly headers and up-pipe if you want to be at 400.
- Dyno tune and AccessPort (AccessPort can be skipped if you run open source and have a tuner familiar with it)
- Seats (stockers are heavy and way too high for most people)
- Steering wheel (unless you want to run OEM which most don't)
- Wiring - Are you going to do it yourself or buy the iWire kit? The iWire kit runs a bit over $2k, or, if doing it yourself, you can spend months to do it cleanly and still spend a few hundred on extra wire, connectors, tape, loom, zip ties, relays, terminals, lugs, etc.