Yes, all improvements and fixes applied to 2.5 are ported to 3.x. So, all fixes in 2.5.8 will be available in 3.1.0.
This brings to mind a question. Not all customers want to be on the bleeding edge of the latest release, and I assume currently Mendix are supporting the code lines for 2.5 and 3.0, and have scheduled maintenance releases for both code lines, but...
I have not seen any official statement about version support. Is 2.4 still being supported? Do Mendix always support the last 2 major releases, the last 3 releases...? At what point is the first answer to any issue, 'Upgrade...'?