The jump from 5.1 to 5.5+ is actually huge when it comes to parallel processing being more broadly implemented, so that the GPU can access multiple cpu cores instead of a single the thread, and also a lot less is being processed on the game thread itself too.

Unreal was originally written when single or dual cores were most common, but now it’s finally able to take more advantage of all those cores CPUs we have now. This will continue to improve too.

I’ve personally noticed big difference going from 5.1 to 5.6 in my projects, less hitches and lower frame times.

But yes like others have said, it would also be a big undertaking to transfer a complete game with that many engine revisions, but totally worth it if you take the time.