i think you need to look again, areasix will do that for £4600-4850.
blacktops seeem to be the way the power is going higher, no idea why, probably a younger engine and solid lifters instead of hydraulic, bigger gearings for cams too.
to be honest it's not that much money to get to 500bhp, but that doesn't say anything about management or cost of turbo or manifolds etc.
in all honesty i'd start with a five cylinder audi turbo engine, easily hit 500bhp with less cost.
money is the only thing stopping anyone trying different things to get more power.