Aerodynamics / Aerodynamic Cost / Aero parts

It's a while since I looked into it, but I think that the problem with mounting the battery in the seat tube is that the BB shell is solid and so there is no way to run the wires internally.

This is the only real problem I can see. The shifters>Junction B cable can be threaded in the same way as the mechanical cables down the downtube, or you could drill a hole in the top of the top tube in the tried and tested fashion and insert them from there. The hole for the front derailleur cable would need to be extended to allow for a Di2 cable to pass through it, but that's not a biggy. The only two problems I can see are that I'm not sure the standard Shimano Di2 battery will fit inside a P2 seattube (not a problem for me as I have a DIY hacked external battery which is thin enough to fit inside the seat post of an S2), and how to get the cable from the seat tube into the Junction B given the absence of any internal connection between the seat tube and the down tube.

I suspect the answer will simply be to drill and extra hole or two.

There's also the issue of whether or not it's possible to insert an internal Junction B box inside the downtube or somewhere else, or whether you'd have to use the external Junction B box which would be sub-optimal.