Yes, if you Terrastallize into one of your two STAB types (assuming you're a dual-type), then your STAB will become 2x instead of 1.5x. So, Future Sight's base power would be 120 x 2 = 240 BP.
While Terastallized, Pokémon receive STAB on moves matching their Tera Type, in addition to moves matching the type(s) they had immediately before Terastallizing. If the Pokémon's Tera Type is the same as one of its original types, moves of that type receive an ×2 power boost instead of the usual ×1.5.
