I think i found my answer. thanks.There is a thread on here somewhere where a fellow who designs intake systems (I cannot remember his username but I think that the thread was called induction) and if I remember correctly the answer is not much difference. I think his comment was something like 10bhp on a high power car. It sounded like a fair trade-off for a solid system which filters properly.
The late model R33 and R34 GTR boxes have a snorkle from the passenger side guard. There is abosultely nothing wrong with the airbox.
Thats 632 RWHP (at the wheels), not some theoretical % loss through the drivline. Thats through a set of -5's. If it was so restrictive, it wouldn't have made that.
OP, leave the airbox in place. Spend the money elsewhere on the car that actually makes a difference like your twin turbo pipe for example.