Not TMK but I use 50gr SBK and 52gr SMK and don't notice a marked difference on small game, depending on distance. Setting out at around 3400fps their weight and speed on lightly built critters overwhelms the different expansion rates.
Out of interest I sectioned one of each, the jackets measure the same thickness and they differ only at the forward section, the MK core having 4mm longer solid lead than the BK with the BK core set back and cup shaped to accept the plastic tip stem. The BK jacket also has a much bigger hollow point to accept the plastic stem.
Sierra advertising say the BK is more lightly jacketed, if it is the calipers don't show it up on these two projectiles
