Enable for admin search only

Yes. Unhook these filters:

add_filter( 'posts_search', 'relevanssi_light_posts_search', 10, 2 );
add_filter( 'posts_search_orderby', 'relevanssi_light_posts_search_orderby', 10, 2 );
add_filter( 'posts_request', 'relevanssi_light_posts_request', 10, 2 );

These are added on init on priority 10, so adding an action on init priority 11 that removes these filters when necessary should do the trick.

The tricky part here is that since the Fibo Search is an ajax search, it probably appears like an admin search, so you need to figure out somehow whether you’re doing a real admin search or a Fibo search; I can’t unfortunately help you out with that, but look at the global $wp_query for differences you can use to see where you are.

Excellent – thanks! I had just this minute opened the source code to look at the hooks being used to see if these could be deregistered.

My focus is on the product listings page so should be able to deregister them in general but not when on the admin listing pages.

As Fibo has a hook when initiating a search, I added this to functions:

// Manage search - Relevanssi for admin, Fibo for frontend
add_filter( 'dgwt/wcas/helpers/is_search_query', 'motor_manage_search' );
function motor_manage_search( $query_object )
{
    // Remove Relevanssi hooks
    remove_filter( 'posts_search', 'relevanssi_light_posts_search');
    remove_filter( 'posts_search_orderby', 'relevanssi_light_posts_search_orderby');
    remove_filter( 'posts_request', 'relevanssi_light_posts_request');
}

Works a treat! Much faster search front and back now.