I will agree with Phil on this.
There has been no limit on the aspect of "budget"
And so, with no limit on budget, I'll try to contribute to this thread from yet another different angle.
Weight:- To compete against the BR-Z or 86GT on weight and with heaps of moolah, perhaps I would...
* replace the s/steel exhaust with a Kaki Mega N1 Titanium
* replace existing glass with a thinner gauge like what's on an M3 CSL
* replace steel guards and bonnet with aluminium and c/f
* replace existing strut brace with Nismo titanium
* further reduce unsprung weight by getting light Enkei rims
* get rid of mats and soundproofing
* remove a/c
* remove speakers and head unit
* remove rear wiper
* did you know that child restraint brackets weigh more than a Kg?
* and I haven't enquiried yet how much you weigh, to help power/wt