When I got custom molds I believe they were 2.5 mm in my better right ear, and 1.3 in my poorer ear. I got essentially zero feedback with them. However, I could not stand them in my ear. Especially when I was eating they would move and make noise. After one more refit with an open mouth, they were no better, and while I lasted about 3 months I gave up on them. I went back to Rexton closed sleeves in both ears. They are rated at 1.6 mm equivalent vent size. However, the real venting is the sum of the intentional vent plus any leakage vent. The sleeves obviously leak, as they don’t give me the same feedback protection as the custom molds did.
The custom molds did give me better isolation from noise. When the hearing aid automatic noise reduction kicked in, things got really quiet. This would happen when on the highway with a very smooth road. Almost eerie quiet. If someone spoke in the car however, they would immediately pick it up.