1. How are we going to handle pg_upgrade? It would be nice to be able to read the old page format, or convert on-the-fly. OTOH, if it gets too complicated, might not be worth it. The indexes are much smaller with the patch, so anyone using GIN probably wants to rebuild them anyway, sooner or later. Still, I'd like to give it a shot.
2. The patch introduces a small fixed 32-entry index into the packed items. Is that an optimal number?
3. I'd like to see some performance testing of insertions, deletions, and vacuum. I suspect that maintaining the 32-entry index might be fairly expensive, as it's rewritten on every update to a leaf page.
It appears that maintaining 32-entry index is really expensive because it required re-decoding whole page. This issue is fixed in attached version of patch by introducing incremental updating of that index. Benchmarks will be posted later today.