I tried Quad boxing one of the lvl 50 instances with a lvl 73 team (lvl73 mobs). I wiped in a matter of seconds after the first pull. The mobs are tougher than normal mobs, ie - they hit harder and take more hits to drop.

I just bought all the hardware I need for my 6-box team... Hopefully things will have been 'tweaked' by the time I get the two new members to lvl 50... Otherwise, looks like we finally have a real challenge on our hands!