I updated my dev install of XF to TinyMCE 3.4.7 (seems like you can just drop in the new version without any problems). I haven't done extensive testing, but the stripping of tabs within WYSIWYG mode isn't fixed (that's the one that bugs me the most).
Then I tested editing a post, and the existing post definitely isn't going away, although the edit window is getting huge heights, scrolling it off the window. The IFRAME has a 6925px height on a post that is 5 lines. And ~1300px for one that is a single line. But the contents of the post itself DOES stay just fine.
Not really sure it's worth the effort of upgrading it yourself anyway... I only did it in the hopes that the tab stripping in WebKit browsers was fixed, but it's not, so...
Forgotten one more feature: let user to switch between ordinary editor window at the bottom of the page - and floating window (position:fixed), which is very convenient when making several quotes. This is already done for vBulleting 3.7, SMF 2.0 and IPB 2.x, no need to invent bicycle.