I have an older EXC without fan and most of my riding is/was (more 'was' lately) single track and it never overheated - the rider on the other hand overheated on a regular basis...
I'm pretty sure the "6-Days" edition of the 10 comes with a fan, and possibly all models of the 11. I put a fan on my 08EXC and it made a huge difference. Tight, long, windy hill climbs would always have the thing spitting a little bit of coolant, fan fixed it right up. It's not too expensive and easy to install depending on what rad-guards you have
As soon as I bought the bike I also re-jetted with JD Jetting, removed all the emissions crap, and cut out the exhaust restrictor. All stuff the EXC needed but I don't think the XCW did. Stupid California emissions crap
It thing the fan kit is around $100, and it's very easy to install. It's well worth it if you do the technical single track stuff and steep climbs. It's really nice to have if you ever happen to get stuck in traffic somewhere too.