Dave,
If you look at the chart I linked to, you'll see that the density of nitrous varies tremendously with temperature. As Edward said, it is not much like water.
Gerald
PS - If you increase the pressure of the supply tank, then yes the flight tank should fill faster. There will be some pressure loss through the fill line because of resistance to the nitrous flow. There will be some fractional pressure drop at the flight tank through the vent line. The net result though will be higher pressure in the flight tank. Higher pressure yields higher equilibrium temperature and lower equilibrium density (per the chart I posted). The motor will have less total mass of nitrous in the flight tank but will run at a higher pressure.
The net result is changes in supply tank pressure end up altering the burn curve of the motor - thrust and duration, and total impulse. That's for a classically vented flight tank. You want to keep the nitrous supply tank pressure within the 650 - 900 psi range (verify via gauge) to stay within reasonable range. Too low and the fill will take a long time and it is possible the flight tank won't have enough pressure for reasonable flow through the nozzle. I could give some numbers but it is likely out of bounds for this forum. Too high a pressure makes nitrous much more dangerous - it IS a monopropellant after all. Higher temperature greatly decreases the energy required to make it go boom. So stay in the 650-900psi range, which is also the range the motor should have been certified for.
EX of course, would be up to you and the RSO.