I guess I'd second Harry's somewhat ambivalent observations. I don't believe that there's a solid concensus, and "beauty is in the eye of the builder", as it were.
I'm sure that the boat tail can be built as light as the conventional cowl shape. The advantage of using the conventional shape when I built eons ago was that one could buy them 'pre-built' (with plenty more work still to do, of course). Fabricating a cowl from scratch is a non-trivial task, so I'm with Harry in the 'energy department' - some day it might happen... :?
As best I understand the experience of others, there generally hasn't been any aerodynamic downside to the boat tail in comparison to the conventional shape, and it seems likely to generate less drag; but I'm not aware of any spectacular gains from people that simply switched shapes either? I'd like to hear some real numbers from others who have actually made the switch. Klaus Savier uses a boat tail shape on his VariEze, which tells you something.
Going a bit farther out on a limb, I would speculate that it is important in shaping the boat tail to have the rise angle on the bottom of the cowl aerodynamically match the convergence angle of the cowl sides, assuming that the boat tail has a semi-rectangular cross section (i.e., rounded corners). This idea is based on optimum wheel pant shape where it has been determined that you don't want the flow to have any tendency to wrap around the pant in the process of flowing aft. Another way of saying this is that you want the static pressure to be constant around the cowl at a given station location so that the air doesn't try to flow 'around the corner' in either direction (i.e., side to bottom or bottom to side).
I believe that the seven degree angle comes from old NACA research studies on optimum divergent-flow duct shapes; but it should provide a reasonable estimate for external flow applications too.
