I just did the buy, try, sell thing with an MXC tank. AAMOF, the MXC tank is in a large box in the back of my 'burb on it's way to the post office this morning to go to a better home.
I thought it'd be okay after I installed it. Didn't feel bad at all on the stand or riding around the yard. On the first real test ride on some tight single track, I was able to ride a buddy's sister bike back to back with mine. Big difference to me. Much narrower at the tank/seat junction and easier to stay forward on the EXC tank.
If I was not racing and/or had much longer/more open rides, I'd have proabably kept the tank. It's a good design but just too wide for me in the tight woods. The knee braces on each knee probably exaggerate the problem a bit for me, too. Not sure if I'll get the IMS tank or not. I'll wait for some more reviews.