Nistune Boost control is actually very good, you have the option of a 3D table.
wow.. a RB25 coming on at 5~5.5k with only 500hp rear is pretty terrible.
Question around your manifold setup, is it divided or open? Please post up a photo of your hotside. Also cams won't bring on your boost any sooner, I would look at seeing if your VCT is working, without that you're already sacrificing 500~600rpm of spool time.
(bragging rights) I was doing 499hp at the rear on a Mainline dyno with semi-slick tyres. Hit 1.65bar by 4100rpm.