No stats and that doesnt suprise me.
PC game demos have been in decline for a while. In fact, the trend started right about the same time games started to get really overpriced and suck real bad.
I think the publishers have stats on such things but they wont admit to them. If a game just plain stinks or isnt very entertaining then the demo will actually take away sales. But it could be that many folks who really wanted to play the game will probably buy it anyway and are just using the demo as a sneak peak. If this is true (which is what I believe) then demos only have the potential to hurt sales.
Most publishers these days seem to rely on being able to put out crap, make a little profit off all the people who rush to buy it immediately, and then move on to their next project. Thats why we usually only see one or no patches for a game these days. Patches really dont increase the potential for future sales because the folks who really wanted the game probably went ahead and got it even if they heard it had issues.
Only online games get regular patches because they need to keep making money off it every month.
I'm still waiting for a remake of KOTOR 2, because I know that fresh new sales is the only reason for them to fix all the bugs with the engine itself and also redo the story properly. As for just a patch that makes it work well with all modern hardware and OS's, that will never happen.
No money in it.
Getting back to demos: Most of the sales they get come from hype & marketing, not the good word from folks who actually played the game. Once its been out in stores and they sold a bunch of copies they generally arent putting in much more effort.