Thank you for your questions!
1. Does it work with HyperDB plugin
Yes. ClouSE plugs into MySQL on the backend and is transparent to all tools and technologies that work on top of MySQL.
2. Will it work with plugins that create new tables on installation
It would work with most such plugins. You can set ClouSE as the default MySQL engine (
default-storage-engine=ClouSE in my.cnf), then most plugins would just work automatically. Some plugins specify the engine explicitly when they create tables, so they may create tables in a different engine. WP2Cloud would warn about such tables and you can manually change them to use ClouSE (using
ALTER table ENGINE=CLOUSE); statement).
3. What is a overhead of maintaining a connection with S3 in terms of transfer and requests
It will depend on the usage pattern. ClouSE heavily relies on compression, caching and batching to reduce the number of requests and amount of transferred data.
4. How big is a working set of data on instance in relation to the full database size
ClouSE tries to cache as much as possible given the amount of resources on the instance. Most frequently used parts of data are more likely to stay in cache, and less frequently used parts of data are more likely to get evicted.
Hope this helps.