OK, I think I have the optimal Up+Down TAC combo down.
(corner) Up or Down TAC, sj.M>H>L Smart Bomb, M>L Smart Bomb, L>dH>fly, M>H>L Smart Bomb, L>H>L Smart Bomb, (fly ends) L>H>ADU, H>dH>S, land, H Smart Bomb>Proton Cannon.
On some characters, this variation is much easier:
(corner) Up or Down TAC, sj.M>H>L Smart Bomb, M>L Smart Bomb, L>H>fly, M>H>L Smart Bomb, L>H>L Smart Bomb, (fly ends) L>ADU, L>dH>S, land, H Smart Bomb>Proton Cannon.
At the beginning of the TAC, you MUST hit j.M as early, and delay the j.H so that it hits the opponent as soon as Iron man reaches the apex of the TAC jump. You can replace most of the j.L in a smart bomb chain to j.M, but the execution gets that much tighter, and you’re only adding like 15k damage.
Be most cautious with the delayed j.H, and the L smartbomb link to j.M>Smart bomb chain. Those have the tightest timing in the combo, at least IMO.
On a side note: with some adjustments I believe Iron man, from a down TAC, is able to link into a L Repulsor after landing. The execution’s really damn tight, but I’m hoping it can be possible.