I'm leaving on anything up to 10PSI as long as the track will hold it (well could go to I think about 16PSI but that's driveshaft twisting/input shaft breaking territory or it just does a burnout on the line....) - so I generally don't need to wait for boost once I've left. Usually leaving at about 2,800rpm and try and hold 2-4PSI.
I feel like if I change early from 1st to 2nd early, the change is quite long and the decceleration jolt as it changes is quite severe.