We are currently evaluating the feasibility of replacing Ajax Search Pro SQL queries with WPSOLR Solr/Elasticsearch queries.
There are some discrepancies to be filled though. For instance, Ajax Search Pro returns CPTs (custom post types), as WPSOLR does. But it also returns other object types (like users, terms, categories, comments), but WPSOLR does not.
After a lot of digging, I decided not going further with the Ajax Search Pro add-on.
The code to build SQL queries by Ajax Search Pro is very complex, as expected, and cannot be mimicked in its current state without cloning all classes in /ajax-search-pro/includes/classes/search/*
Or it would require to add many apply_filters() in Ajax Search Pro’s code, one for each part of the SQL.