IDK if the sheet film cutting equipment can be adjusted to feed 5 mil at the same speed or at all. There are a lot of factors involved in this, but coating is the least problem.
In processing, removing rem-jet is more difficult with sheet film as well.
Flatness in a holder, is, of course of very great concern and that is why Kodak sheet films are coated on 7 mil support.
The list of problems would mount, I'm sure, if I kept thinking about this.