X-Plane 850 is now beta - see item 7 of Klingon Software Development. A few random thoughts:
We aim to make each new patch of X-Plane "performance positive", meaning for the same settings as the previous version, the new version will run faster. X-Plane 850 is definitely not performance positive right now, but this is mainly because it is an early beta. Expect lower framerates until we have time to do some tuning work and analysis.
X-Plane 850 supports a new apt.dat revision with curved taxiways, lines, etc. However you will not see this in the betas. The reason: the sim supports the data, but we have not yet shipped any new layouts using these features. Until custom scenery authors and layout creators adopt the new features you won't see them in the sim, even though the capability is there. Because we have not yet released editing tools for these new features, it'll be a bit before people can take advantage of them. (You can make curved taxiways now by reading Robin's spec and then using a text editor, but this is a very slow and painful way to make a layout. This is how I have to make taxiways to test the code right now.)
Finally perhaps most important: Austin and I do not have time to follow all of the various on-line forums and communities, so if you post a bug to a forum but don't file a bug report with us, the bug probably won't get fixed! Please use our bug report web page to file bugs with the new beta.
(The bug report web page is for bug reports only. In particular, if you bought X-Plane and are having trouble with it, please email info at x-plane dot com to receive tech support. Filing bugs will probably not get you timely tech support, but emailing our tech support people will.)