A fair amount I would suggest especially if you pay someone to do it.
It's beyond a stage 1 tune.
Plus I'd think it would be wise to add costs for suspension, brakes, clutch (defo) at that sort of power.
I like all Skylines.
I bought a 33 GTST for a few reasons. One it was stock and clean. A stock car (assuming it has not been modded then de-modded) is more likely to not be on the verge of mechanical meltdown. Another is it's my first 'fast' car and I didn't want to spend the extra 10 k say that a good 34 GTT would cost. Maybe I'd not really enjoy it that much (wrong anyway). Another reason for getting stock is that with 'only' ~130 kw at the rw's I'd be less likely to have an upset and crash etc. Not that I am a poor driver but I'm old enough to know I'm inexperienced with HPV's and it is only prudent to be like that IMHO.
I wasn't considering a 32 GTR 'cos of the power for that reason, even though I like the look...and they are a GTR. Also I needed a daily drive and was wary of the complexity which implies more servicing etc. I like the look of the 33 S2 even though some don't....I like that long(ish), lowdown, look...wide. It looks longer than it is of course, but it has that presence like say a Soarer...the long(ish) bonnet, mean, muscley, it's really only a medium sized car of course though. The inside is fairly roomy, doesn't feel cramped...feels expansive in the front seats. Yeah the 34 looks great I think also, more aggressively styled but less like an archetypal 'tough' car IMHO which is what the 33 looks like...the trade off is that the 33 is a bit more anonymous.