The new sound proofing works about as well as the old sound proofing, to my ears.
If lots of people want to get down to the Cave, one person could pop through the cafe, open the back door and let everyone and their equipment in through there. It is probably easier than fighting through the customers and chairs, and most bands seem to leave this way anyway.
I like it to be nice and quiet on Film Night, and on the whole, bands aren't too loud on Mondays. Could we suggest that quiet events favour certain evenings, and then ask the loudest bands to favour other evenings for rehearsals? No one has exclusive use of the Forest any time they want, so the quiet events and loud rehearsals should work out how not to clash. 2-3 evenings a week aren't much for the Cave to be closed to very loud bands.