The boosts are multiplied successively, in the sense that it is BPx1.3x1.5. I've done calcs for Life Orb damage, and the combination of Sheer Force + LO on an attack is a 69% boost.
Flamethrower in Gen 5 does 160.55 (161) damage without factoring in STAB if used with Sheer Force + LO.
With Bill's reasoning (BPx1.6 in this case), it does 152 without STAB. This is only a 60% gain.
With Mega Pinsir, it will do BPx1.3x1.5 with Aerilate + STAB. For example, Quick Attack will go up to 78 BP, and Return (max) is 198.9 (199). Both of these back up Jay's 95% claim.
tl;dr Jay and TPO3 are right.
~AoH