From 2.9.13 and Premium 1.7.6 there’s a new filter, relevanssi_results.
This filter is given an array containing the search results in (post ID, weight) format. The filter expects to get the same thing back.
This filter can be used for processing the hits somehow, modifying the weights and whatever you can come up with. I suspect relevanssi_hits_filter is going to be more useful in most cases.