So last night I unlocked and completed level 9-1. So I now have all the K-O-N-G letters and all the puzzle pieces. I went to view the image gallery, since I have collected all the puzzle pieces, and I noticed that the "Golden Temple" gallery has a picture of level 9-1, and then lots of other levels I've never seen: a minecart level wrapping around a giant banana, a Super Paper Mario style level, and a Virtual Boy level, among others. Are these real levels that are unlocked by completing enough mirror mode or time trial levels? It seems weird that there would only be one level in world 9. I've checked the three FAQs on GameFAQs and none mention any levels beyond 9-1, but they all appear to be incomplete too.
If I had to venture a guess, I would say no, there are no more hidden levels except for 9-1, but I don't think anybody knows.
The only way to find out is to re-beat the game in mirror mode, collecting all the puzzle pieces and K•O•N•G letters again. To be double sure, you'll need to get time attack medals on all regular and mirror levels. To be triple sure, those should probably be gold time attack medals, or maybe even shiny gold medals.
I'd volunteer myself, but would like to keep my job, marriage, and family.
I'd really like one of the game's creator to speak out on this issue, but don't think that's likely to ever happen.