Homasote over birch ply here also. I have some cookie cutter sub-roadbed completely covered by homasote, and I have some sections fully covered with the plywood but the homasote on top is cookie cutter. I've noticed the former is much quieter than the latter so at some point I'll go back and glue down pieces in the uncovered spaces as part of scenery.
If noise becomes an issue with the foam tops it may be possible to attach something to the underside to quiet things down. Homasote if it is available, auto parts stores carry Dynamat or some other dampening material. I don't know if ceiling tiles are dense enough to make much difference but that could be worth a try.
