Might be able to do 3PP > launch though.
No. In fact, that was the first thing I tested. The result was any hit on a back turned opponent in a sit down stun results in a knockdown, an immediate knockdown at that. There is not threshold, there is not guessing, they just drop. You can test this by doing the Tsuji-Kage yourself (41236T) then walking around the opponent or by doing 46H which sets up the situation immediately (BT SD stun).
The upside to this is receiving the BT launch height on any strike you do (as it would be with Kasumi's H+K to 6K, or H+K to any launcher at the wall that fits). That means the possibility is that you could dash behind someone then launch them with 8K/6P+K~P+K, resulting in the highest launch height possible straight into 4p6p+k~k, bt 7k.
You're right, Ultimate could be different though, but the reality is I doubt that TN would be this giving of guaranteed damage like that so the situation is most likely the same. That being said, if 41236T C!, Tele, 6p+k~p+k works on highest SE, this means that mids and heavies will always be vulnerable to max launch combos (i.e. Instead of only getting 8p, pp6pk off of the immediate launch, 4p6p+k~k, bt 7k will always be available.