1. According to the manufacturers, no one should load 150 grains of loose 777 powder into a muzzle loader that's rated for 150 grains because loose 777 needs to be reduced by 15% volume.
Only the 150 grain equivalent 777 pellets can be loaded into a gun rated for 150 grains of powder.
2. The maximum load is not necessarily the most accurate load. Some bullets are designed to be pushed fast and some aren't.
For example there are Hornady XTP's [pistol bullets] and Hornady XTP Mag's that have different expansion rates.
3. According to this generic Traditions chart, the difference between 100 grains of powder and 150 grains is about 250 feet per second with a trajectory difference of about +2 inches at 200 yards. There is a very noticiable difference in energy.
http://www.traditionsfirearms.com/loadchart.pdf
Loading more powder depends on the accuracy, type of terrain and possible shooting distance, size of the game animals, whether good quality bullets are being loaded, whether a ballistic bridge sub-base is needed to hit the target or not and if the felt recoil is tolerable enough to not flinch at the shot.
The more powder that's loaded means more powder residue after each shot that may need to be swabbed before reloading. In the field, a 2nd follow up shot may require using a different projectile that loads easier like a Powerbelt bullet.
Ballistic bridge sub-bases are more often used in the Savage smokeless powder guns, but if accuracy suffers with maximum loads then maybe these will help. See what they're about near the bottom of this linked page.
http://www.mmpsabots.com/