Raimey,
Based on the results stated here by some of the low pressure loaders that use very slow powders, I'd say it's likely the low end pressure/efficiency issue is dependant upon the burn rate of the powder. I've been using a bit faster powder than some others but loading to similar pressures and velocities and have noted relatively good powder burn, albeit not as clean as a higher pressure load using this powder (Hodgden Clays).