If you have a UK spec R34, presumably your car has warranty... Why not keep that intact and get the middlehurst 400bhp upgrade (cams, remap, exhaust, ?induction), costs £2k+Vat I think and you get to keep your warranty too...
Would like some definitive figures to be able to have the balance off though. ie Mines don't change it upto 600bhp. I mean how the hell do we know whats been done, and what hasn't been done, to these 'Factory' jap cars and also have a car dyno'd with and without I/C. Also doesn't an upgrade I/C come into its own on hot days. But then doesn't I suppose in winter ?
Suppose till someone proves it for sure we'll have to keep guessing.
The reason a lot of people tend to skip changing the IC in R34s is that by fitting an item which is at least twice the size of the standard one you are 1) reducing the air flow to the radiator and 2)warming up the air that gets to it.
Some prefer to keep the old RB(which produces a lot of heat) running at optimun temps, and usually trade off a marginal HP gain to mentain good airflow.
There is also the argument that with the lower octane fuel in the UK an IC will keep det in check due to the 1-2 degs cooler air entering the engine.
The final argument, favoured by Geln , is the fact that nothing quite looks as good as a huge IC right against the front bumper.....