How much faster is the Pro version with multithreading and GPU acceleration now?
Is it enhanced in comparison to Beta?
No the speed between beta and PRO should be the same (although there was 1 beta version that's significantly slower due to a bug, but I can't remember which build). The incentive to upgrade is to get the latest features/fixes. If you're happy with the beta's features/bugs, then there's no reason to upgrade.
Depends on the solver and the scene. GPU takes over when it can or needs to and vice versa.