One small caution....drill through with the small bit at the desired spots. Then use a collar to set the depth on the drill bit for the larger relief hole. You can guess why I know that. Any type of collar will work, even rolled masking tape.

There is no down side I can think of, unless you don't have fairly easy access to the back of the firewall to install blind nuts....then it could be a pain. BTW, 3 or 4 4-40 bolts is all you need to mount the soft mount to the firewall, but 6-32 is probably easier to work with.

These mounts are a little stiffer than the commercial units (assuming you can even buy a commercial unit for the SAP engine) but the nose ring is not necessarily a must. I've done multiple snaps with my test bed and the mount damps very quickly. The stiffness has not hurt my servos in many hundreds of flight. In my primary plane I am still using original servos with a similar mount up front

I've seen commercial units allow engine flop all over the place on the ground when the engine's nose wasn't supported, simply by revving the engine up and down.

Another BTW, 1/8 A/C ply is okay to use. I use 3/32" or 1/8" normally. On the spacer, if it's firm balsa you're fine.

Matt