In order to properly evaluate elasticsearch we need to be able to run the same query against the same dataset using each engine to compare results. Ideally we can keep both innodb and elastic indexes updated in parallel and switch backends using a request parameter or feature flag.
Yes we can.
When elasticsearch is configured, phab uses it, when it's not configured it uses mysql. So we can run a separate indexer with PHABRICATOR_ENV=elasticsearch which will index into elastic.
Then we just need something like a feature flag to trigger the front end to query elasticsearch on demand (the feature flag is something I can easily implement in the phabricator request router)