I don't pretend to be an expert, but I don't mind telling my limited experiences with muzzle loader loads.
First of all, 90 grains of 777 is the equivilent of just over a 100 grains of black powder or Pyrodex. Like you, I've tried more powder and found that accuracy deteriorated.
I'm sure it varies from gun to gun, but in my CVA's (a bolt action and break action inlines), I have found that 90 grains (by volume) of Pyrodex or 80 grains of 777 gives the best performance with the added benefit of not being very punishing to shoot. I'm pushing a 370 grain Maxiball and it definitely packs the punch necessary to drop a deer at the ranges you're likely to encounter in deer woods.
To be honest, I've never even considered shooting 150 grains of anything. I know that these newer guns a built to take it and I'm sure that some people have success with it, but that's just not necessary for me and my hunting conditions.
Now take this note and $5 to Opryland Hotel and you can have a cup of coffee. :smile: