It's possible to reduce the size further, one of our devs has recently been experimenting about this in particular and I think has got down to 4MB without too much difficulty. The problem is that reducing it much further starts to mean being more careful about blacklisting stuff like modules in the python standard library, which can be hard to do automatically without breaking dependencies. It's highligted some potential optimisations in kivy's internals though, so maybe we can make it easier to leave out some non-essential kivy components.
Just to be clear about the build time thing, compiling python + project dependencies may take a few minutes (e.g. your 15 minute figure), but subsequent builds don't need to repeat this and will take just a few seconds.