According to Hodgen, loose 777 is more potent than the 777 pellets so you can use less powder to obtain the same velocity:
50 CALIBER POWDER MEASURE SETTING TRIPLE SEVEN FFG (loose powder)
240 Gr. Hdy. XTP 50/44
70 GR. 80 GR. 90 GR. 100 GR.
1633 1703 1806 1883
-----------------
240 Gr. Hdy. XTP 50/44 with pellets
80 grains One 50 cal. / 50 gr. + One 50 cal. / 30 gr. 1613
90 grains Three 50 cal. / 30 gr. 1691
100 grains Two 50 cal. / 50 gr. 1801
Data:
http://www.hodgdon.com/data/muzzleloading/granular/t7rifle-sabot.php
http://www.hodgdon.com/data/muzzleloading/pellets/50sabot.php
About using bore butter with sabots, some people use it very, very sparingly to help with loading and to keep the first shot on target, but this might depend on the temperature and your individual rifle's preferences. Using it is not recommended by TC since it could slightly elevate barrel pressure and might even cause worse accuracy for some rifles
When I've used it, I swabbed with a minute amount in the top half of the barrel only after the powder was dropped but before the sabot was loaded, Not that it really matters, it's just that I didn't want the loose powder to stick to the inside of the lubricated barrel.