After my install..I would tend to agree that the FMB's might dig into drywall (even if mounted directly into a stud), just based on the general design of the bracket and where it mounts to the speaker.
You could TRY mounting using molly bolt drywall screws, but I would recommend against it..I would want a stud to screw into for peace of mind.
If you look closely at my setup, you will see that my FMBs mount to a block of wood approx. .375" thick..the FMb is thru-bolted to the wall with drywall screws.
other than that, it looks like you have some drywall/carpentry work in your near future! No problem!! (cant see it from here