I just took delivery of my new W520 a few days ago and it came with Windows 10 Pro. I've got 3 other machines running FreeBSD but am keeping Windows on this one so I can play some of my older games.
I had seen people talking about how the W520 could run hot but was surprised how cool mine ran overall and while gaming. That's The Elder Scrolls IV - Oblivion it's running now. I have no idea what the frame rate is but it looks great with no signs of lag. I'm planing to get Skyrim but have barely scratched the surface of Oblivion.
While waiting for it to be delivered I was looking for cooling pads when I saw these Opolar cooling fans on ebay and decided to try one out:http://www.opolar.com/shop/laptop-coole ... ling-pads/http://s28.postimg.org/d8tujk1cd/520gaming2.png
It defaults to an auto setting that spins the fan down when air drawn from inside the laptop drops to 89F, and again at 88F, but really does better with a manual setting slightly above that. It does make sense from the point of drawing air directly though the case as opposed to a fan blowing air up at the bottom of the case, and you can see the temp dropping on the display and if you can monitor it from the desktop.
Overall, the W520 doesn't run hot enough to warrant it (beside the fact of it being kind of bulky), and is better utilized cooling my Sony Vaio while it's compiling ports.
Windows 10 runs fine on my W520. I'd rather have got Windows 7 if I had to choose, but have finally got 10 tweaked to my satisfaction. If it wasn't for gaming though I'd wipe Windows in a heartbeat in favor of FreeBSD.