When I got my bengal I wondered the same thing. I wanted a rosetted cat, but thought Kai's spots would stay solid. However, over the first year of his life his coat has really changed.
See my thread here for pictures of Kai and other people's kitties and their development:
viewtopic.php?f=2&t=19948I think if you are looking for big doughnut style rosettes, you should keep your eyes peeled for a kitten that already has rosettes right from the get-go. Kai has open arrowhead and pawprint rosettes, and it has been interesting to see them develop from strictly black spots to tricolored rosettes.
I can tell you with confidence that going with your 'gut' is a good way to go! Kai didn't have the "looks" I wanted at first, but he had the personality and temperament....and I couldn't have made a better choice!! He has grown to be extremely handsome as well as totally loveable and friendly. Good luck!