I've seen you overtook the aerodynamic stage of you project, but playing yesterday with a flow simulator (which is approximation and even misleading) I have seen that the flow on the back of 316 is not so bad (compared with my 2008 focus hatchback) and the turbulence on the back in case of your kamm is amplified.
A trunk lip (quite long I will say) may do a better job. Something to test if you'll like.
