The plasma gun on maximal does 19 damage on average. I am using the BC stats because that's what I have in front of me. I am assuming that the TT gun is firing using that mode.
Even with 19 points of damage, we are still in the S6 category (right above the Heavy Bolter (S5) and next to the Multilaser (S6)). And as I have mentione,d I don't think that the TT plasma gun is supposed to represent maximal-mode shots, mostly because Recharge and blast increment (in the TT, some plasma weapons have the ability to increase their blast radius with an "overcharge" mode, that is probably a Maximal equivalent, but plasma guns don't have this option).