If you want to use full-bore instead of sabots, I would also recommend the FPB - they have them in a 300 grain version now as well as the 350. The Thors are good bullets as well, but more expensive. Both options will allow you to use heavier powder charges and perform well.

However, I wouldn't use 150 grains of loose 777 - that is a pretty powerful load, not recommended for CVA muzzleloaders. (not to mention it will not the snot out of you with a 300 grain bullet) I wouldn't use more than 120 grains of loose 777 or 150 grains of pellets.

As far as accuracy goes, there's no way to tell until you get out to the range and try some different combinations out.

Powerbelts are another full-bore option to use, but I would only use them with light to medium powder charges - they have a bad reputation for fragmenting at magnum powder charges....