Zapp Branniglenn
Even accounting for the 1 frame leeway, couldn't we actually look at -9 to -7 depending what the fastest invulnerable option or hitbox is? Since grabs ignore shields anyway, there's actually marginally more room to punish with them. There aren't any frame 1 rolls or spot dodges. There are a few frame 1 attacks though, so that counts too. Of course, this is assuming you can get the frame perfect punish consistently. So it might round out to -11 anyway if you're allowing for the most leeway.
That being said, I still believe that looking for other options is a good idea. I may have mentioned this before, but Ftilt and B-air OOS take the exact same number of frames, so anything you can punish with Ftilt can also be punished with B-air, provided the target isn't squatting or too short. I've been gradually working B-air OOS into my game as well, and it's helped me take a few stocks in crucial moments.
Also for moves that link into other moves, do you have the advantage for linking into those moves on shield? For instance, a character might have a single jab that's -5 on block but then it might be -3 when linking to jab 2 if that makes sense.
In Bowser's case, Dtilt 1 to Dtilt 2 on shield is +0, making it a true blockstring with no gaps.