Where's the limitation you're hitting? You mention the searches are failing, but are we talking about failing because of timeout? How are they failing? If your search challenge is based on hardware, have you considered another search head or indexer?
Not really, and if your problem is that you have reached a max, prioritization won't help because existing searches are not going to be killed off to make room for a new one one that comes later, even if the new one is to be given a higher priority. You can solve your problem with a combination of:
raising the max concurrent searches limit. if you have cpu, memory, and IOPs to spare when you are the max, then you may increase the max.
limiting all users to a lower role-based concurrent max.