I was adding up a mods list last night and it came in under $5K
K&N panel in stock airbox - $100 - here
3" turbo back exhaust - $700 - here
Turbotech manual boost controller - $40 - here
Boost gauge - $40 - any auto store
Generic FMIC - $100 - here
Piping kit - $230 - here
Fuel pump - $180 - here
PowerFC w/ controller - $950 - here
Tune - $600
Total so far: $2940
Clutch $320 here + $300 fitting unless you can DIY.
Front and rear stabiliser bars - $460 - here
DBA rotors - $450 - here
Pads - ~$100
Total: $4570
Most of those prices include delivery,. The parts listed are all pretty easy to fit if you have a decent set of tools.
Thats $5K for a car that will produce ~200rwkw and turn and stop reliably.
I presume you liked the look of your car to begin with or you wouldn't have bought it, so why not leave it that way and avoid unwanted attention. And as far as the stereo goes, I'd rather listen to my car most days.
edit: throw in a HICAS lock bar from GTRgeoff and a set of pinepples to use up the remainder (bad maths skills)
Pineapples - $60 here
HICAS lock - $150 - here
revised total $4780