I'm expecting some additional vibration... how much, I'll just have to find out. If it's too bad, I've got several choices, including going back to the original engine mounts. Other options include drilling out portions of the urethane to make it softer, or filling with a softer urethane. I'm fairly soft now, but can go quite a bit softer.

This strategy is normally used to firm up the engine for performance reasons. For the track many people go with solid mounts that are terrible for vibration, so this solution is kind of in between normal soft mounts and solid mounts.