We have been running 2.3.1+ in production since August and have been running 2.3.2+ since Oct 5th. Our Moodle installation doesn't have as many users, ~70k, and we run on 1 webserver and 1 DB. Here are more details:
https://moodle.org/mod/forum/discuss.php?d=212411&parent=930374
We have upgraded every August and December to stay current with latest 2.x release. Our upgrade from Moodle 2.2 to Moodle 2.3 did not see performance hits on the server side. We did see a performance change on the client side loading all the JS and AJAX for drag and drop when turning on editing in a course. When turning on editing, the page still loads fast but the javascript takes about 10 secs to finish to the point where you get the message "Drag and drop files onto course sections to upload them." There are tracker issues working on this, for example, MDL-34328, MDL-35569, and MDL-35672.
I beleive the MUC is still waiting integration, see MDL-34224. I would be interested in helping test the MUC. It looks like Tim Barker is working on developing a test procedure for the MUC for 2.4, see MDLTEST-328.