Sub bases will help some by forming a better gas seal, but traditional blackpowder is extremely inefficient. The reason you get more unburned powder with the higher loads is because the powder takes up more of the bore volumetrically, thus some of the charge is farther away from the ignition source and doesn't get burned. Its really nothing to worry about, just realize that the more BP the more inefficient it becomes. The guys shooting 150gr of BP really crack me up since only about 2/3 of that load is getting burned, the rest is contributed to the ejecta and only causes the gun to recoil harder with very little increase in velocity vs the 90-100 gr load.

BTW, If your using sabots instead of powerbelts, I highly recommend MMP sabots. Just throw away whatever plastic crap comes with the bullets and use theres. They are by far the strongest on the market and at 80-100 gr. BP you shouldn't need a subbase