I wonder how much difference is there between the FPS let say with a 300 grain XTP mag:
85 grains of triple 777 2F
110 grains goex 3F
???

I can't hit those exact combinations for you, but I checked my Mustang records and found these which should give you a pretty fair comparison.

The 300 grain XTP over 90 grains T7 FFG in a crush rib sabot gave me 1653 fps (5 shot average).

The 300 grain Gold Dot over 120 grains GOEX FFFg in a crush rib sabot gave me 1641 fps (5 shot average).

I've not done that comparison but based on other bullets of 300gr size and 777 2F tests I would estimate that your 85gr of 777 should be in the 1500 to 1550 fps range. The Goex load should be in the 1600 to 1700fps range +/-. In my testing in at least one rifle I've found minimal differences in velocity between same volumes of777 2F and Pyrodex P (3F) but I've not checked against Goex 3F. Barrel length will have some affect on these estimates, most of my guns have 26" barrels.