The upgrade bug bit again and this time I got Ultegra Di2 mixed with Dura Ace 9000 bits (Cranks, brakes, wheels). The main reason for sticking with Ultegra Di2 rather than 9070 is to keep my 7850 c50 tubs relevant. The lower cost was also a factor, but the big reason I stayed with 6770 is the ability to switch with my other 10sp wheels.
Adding another wheelset to the mix, I grabbed a pair of C24 Tubeless rims wrapped with Specialized Roubaix tubeless tires. Without sealant, they don't ride nearly as nice as a good pair of tubs, but for long sportive events, I'd much rather carry a spare tube or two and some co2 rather than a spare tire.
As far as installation of Di2, my Felt FC has a chainstay mount for the short battery mount, but the cables are still routed externally. I took a drill to the cable stops and popped out the rivets, widened one of the holes for the di2 cable, sanded down the rough patches and covered them with stickers to keep it looking pleasant.
The cable that runs along the downtube between the junction boxes enters at the right cable stop and exits out of the front derailleur cable entry point below the bottom bracket. On this frame, the cable guide below the bottom bracket uses a sort of expander bung to keep it in place. I was not able to just run a bolt through and directly attach the external junction box to the frame, so I had to find a longer bolt that would fit the expander, cut it to size and chased the threads on a grinder. The external junction box sits on top of the plastic cable guide, which actually ended up working better because it gives more room to organize the cables.
I made the mistake of ordering too short of a cable for my front derailleur, so as a temporary solution, I spliced the cable with a 3.5mm audio cable just to get the system running. A proper cable will be on the way soon.
Although tempting, I probably will not run the rear derailleur cable internally, because drilling there is a lot trickier than just slightly widening one of the cable stop rivet holes. I'd be adding a new hole rather than just widening the hole by ~2mm.
The frame is bb30, and the adapter I chose to use was Hawk Racing's BB30.24 bottom bracket. Hawk Racing bearings roll very smoothly, and the bearing set (including the adapter shell they're pressed into) weighs a scant 82 grams.
Still to do: run new brake cables and housing (existing housing from my Force set-up is too short for the front brake, and the cable adjuster is maxed out to provide adequate pull and cable entry angle). Black romin (Romin evo perhaps) saddle.