That would certainly eliminate the most obvious cause of the BC discrepancy. The other possible explanation I can think of is bullet deformation induced by feeding. To test that out, you can try manually ejecting a round after cycling - look for any tip deformation that might compromise ballistic performance. The bullet will almost certainly also have marks on the jacket, but that can also come from ejection so it's hard to independently isolate.
That polymer tip can logically only disguise a massive hollow point in the nose underneath, and that's exactly what it does.
The TMK expands almost immediately upon impact, down to velocities of <1600 FPS. The SMK usually doesn't expand at all and only tumbles and fragments down to velocities of about ~2100 FPS. Jacket on the 77 gr TMK may also be a bit thinner than on the 77 gr SMK.
Sierra can say whatever they like about what the bullet is 'intended' for, it doesn't change what it actually
does (or what it actually
was intended for, for that matter - claiming a bullet wasn't made to expand is one way to dodge Hague-related prohibitions overseas). As it happens in practice, the 77 gr TMK appears to be a superb hunting bullet, though barrier performance is not the best.