Okay, I re-tested your track. The sky still has the same issues (as you are aware of). However, you introduced a few more:
I don't like the shortcut jpenguin mentioned. It isn't hard to drive, but it renders using the way around useless. Who would want to go there if he can be much faster by taking the easy-to-drive shortcut? This gives you an unfair advantage over the AI which does not always take the shortcut (and, if it did, the way around the rock would be kind of superfluous).
Another thing is that you seem to got rid of the way around the acid lake, for you do not see it on driving normally. However, some bonus boxes are still there, and so are the drivelines. I tested it by taking the road around – and was not able to advance, as the bridge blocks the way immediately after the bonus boxes. This needs some more removal work.

Finally, the texture transitions when entering / leaving the castle could be improved. And the track needs some more elements, but it comes along quite nicely.