The Dubro mounts are excellent and worth it in my opinion. I've used them on my TF Cessna 182, and am currently installing one in a GP Extra 300 60 size. Instead of redrilling the firewall, try using an 'adapter firewall' or a piece of 1/4 aircraft ply. Secure it to the existing firewall, then drill new holes for the iso mount.

Right now I've got a Saito 125 swinging a (tip and hub balanced) 16X8 on my UCD 60, and it shakes horribly. At idle, all the control surfaces bounce around. The same engine and prop on my Extra with the iso mount...well lets just say it's night and day. I'm not sure if i would bother with one on an ARF, but after hundreds of hours building a kit, it's a no brainer.