You can be effective out to 150 yrds with some shooting practise.As for 150 gr you may get a bit more, say 175 to 200 yrds.But that is a very long shot with a muzzleloader and I would reserve myself to 150 and under.
I agree with trac209 here, but would add that your accuracy might change somewhat when you go to 150 grains (three pellets). It might improve, or go the other way. Try it to see. The most important factors in determining max useable range are group size and your ability to estimate ranges. By going to 150 grains of powder, your trajectocry will flatten some, BUT NOT ENOUGH to compensate for bad range judgement! Of course, if you have an accurate rangefinder, and time to use it, this problem is eliminated.
You might be able to get better groups by using loose powder,and experimenting to determine the exact amounts you need to use to get the BEST grouping. 3" @ 100 yards is NOT BAD, but a lot of rifles will shoot better than this, if you can find the best powder/bullet combination. Looking for the best load is a part of the fun of muzzleloading. (For me, anyway!!)