Only the loose powder 777 is more powerful (about 15%). Pellets are equal. In other words:
100 grains 777 pellets = 100 grains pyrodex pellets
100 grains 777 powder = 115 grains pyrodex.

You can use 3 pellets of 777 in your gun, but NOT 150 grains of loose powder!

Do a search on using 777 and you will find that most people use loose powder because it is cheaper and easier to customize a load. Most people start at about 85 grains 777 and work up to maybe 110 grains. There is really no reason to go higher in my opinion.

I will be hunting elk this fall with 95 grains 777 and a 405 grain powerbelt. Plenty of power for an elk. As noted earlier, this load is about equivalent to 110 grains of pyrodex.

If you already have pellets, I would start with 100 grains and see what happens. Me, I only shoot loose powder.