I read your question and I did some research. According to an article from Nintendo Universe when asked the same question said, "The game is fully HD, and of course, the Wii doesn't support such output resolution." But they are going to release a bunch of sonic games this year so maybe Nintendo fans will get a separate game. I hope this helped!
Yes, you need the disc to be inside the system after you install it. The game is installed the difference is that the disc won't spin. That games installed load faster and play better overall. Also note, that if the the game was bought from the marketplace it does not need a disc, because it is saved directly to your hard drive.