Aren't the batteries and electric motors driving the grid fins at the top of the booster? That and the entire interstage are gonna get blasted with the thrust plume of three Raptors. Reinforcing them enough that it doesn't affect planned reusability targets could take a bigger bite out of the payload than they get from hot staging.

That was my first thought, or that the header tank up at the top might not like being heated like that.

I'm sure they'll figure out pretty quickly if it works or not.