From what I understand the rifle certainly does need to be designed to use 150 grain magnum loads.
There a lot of folks on the huntingnet.com BP sub-forum that have recently added the Genesis to their collection because of the great value of the deal. Everyone seems to really like the accuracy of the rifle. The drop block hinge may get a little bit sticky after several shots but is easy enough to quickly wipe clean. There was one other minor complaint about the lip of the rifling causing a little difficulty to get a tight fitting sabot started, but that sounded like an isolated or generic complaint that could include various rifles having a false muzzle for sabot alignment.
http://www.huntingnet.com/forum/tt.aspx?forumid=26
Folks rarely need magnum pellets unless they really want to reach out across open space and are not very recoil sensitive. And 777 does create a sharper felt recoil than other powders. There have long been reports about a crud ring forming in the breech being caused by using hot 209 primers with 777 pellets, but not [as much] with the loose 777 powder nor as much of a crud ring with the less hot Winchester 777 primers.
Every rifle and how much powder needs to be loaded is different for each shooter's needs, and based on their gun's performance and choice of projectile.
There's nothing wrong with pellets, but measuring out powder charges can be a more economical and precise way to experiment with different loads and velocities. Once a person learns to measure powder, then they can even use a combination of loose powder with a pellet [on top] to fine tune their load. Shooting with less powder means that there will be less powder residue to swab out of the bore after every shot is fired. The more residue left behind, the tougher it will be to clean and load the next shot.
Since loose powder burns cleaner and is also cheaper and offers more loading options, then there's an advantage for the shooter to use it. The main drawback is convenience, but then loads can be measured out in advance, placed into pill containers and easily dumped into the bore with help from a little plastic kitchen funnel bought at a dollar store. A 2nd follow up shot in the field can be loaded using only pellets and/or a lead conical bullet for reloading ease and quickness.
If shooting with loose 777 powder, just remember that the volume of 777 needs to be reduced by 15% to be equivalent to a given black powder load.