Standard air box is too restrictive.
Just look at the letter box the air has to get through, the pressure drop must be phenominal. Got to let the engine breath with minimum pressure losses both on induction and exhaust.
Is it?
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.
engine bay
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.