Jake,
Super Unleaded ONLY
Running with ordinary UL will cause detonation (knock) and will end up with you having a large bill to replace some melted pistons.
You may even find that poor fuel could be the cause of the hesitation, as the ECU will retard the ignition when it detects knock.
The stock ECU in the R33GTS25T ups the boost from 0.5bar to 0.7 bar at about 4.5k rpm, it changes the cam timing at this point too. This is the point the plugs seem to have a bit of bother maintaining a spark if the gaps are too big.
It may also be that your fuel filter could be blocked, and therefore not supplying a quick enough rise in fuel flow when the boost rises to 0.7bar.