First, I would say that trim tabs are essential for the 22. I owned one without, and one with. The ride is much better. Also some one of the fins--and Permatrims are the best. will also help improve the ride.
As for the composite or foam cores, you can still get water intrusion, which is not a good idea. However, on some of the C Dories there is solid glass at the turn of the bilge--think of it--the balsa core and transom core, not not meet, there will be an area of solid glass between. I have found some C Dories with solid glass--some with core. In either case, drill out first, check and the put in the epoxy. You will not be sorry.
I have never owned or ridden in a 27. They may ride differently than the 25 or 22.