I have some Atlas track directly on 1/2" plywood on Mianne bench work right now. It's definitely louder than with no roadbed, but I think it's still quieter than Realtrax on plywood. I haven't had my Realtrax out for a while though, so memories could be off a little there?
To me it doesn't get too bad until you have a long train going above maybe 25-35 smph, then it gets pretty loud, and of course, more trains more noise. I am using Flexxbed, but I don't have enough to do the whole layout yet, got to order some more here pretty soon so I can get that all installed.
I wouldn't recommend a permanent layout like this, would definitely put something under the track, no matter what kind you have.