Sum, according to Kevin, the dyno tuner, it's not uncommon in dry systems, although I'm not sure why. We were using a full 10lb bottle, at controlled pressure, so I don't think nitrous pressure dropoff was involved.
I checked the charts on my old wet nitrous bike, and the torque curves were parallel til redline, so it might have something to do with DFI sensors, like airbox temp, adding fuel, and the O2 sensor shows a trend towards going rich approaching redline. I'll continue to investigate, but yeah I've noticed and am thinking about it.
I have a WEGO wideband logger that will show whats happening at speed, may be way diff than on the dyno.
Dean