I used battery lights for the 24 hour TT and powered my Garmin with a Power bank. This was quite successful and I could do the same for the PBP, but the Luxos U gives a great light and I can run my GPS from the USB port. I'm a relatively new convert to hub dynamos and can't actually notice any drag from mine whilst riding.
Being able to see better helps me go faster at night. I have managed with the very worst battery lights in the past and still don't have any really good ones. The ones I used for the 24 were adequate for main roads, but not good enough for laney descents. As for power output dropping off towards the end of the ride, I was 8 hours quicker on the second half of the last PBP so I guess my power output was higher. I just need to work on getting to Brest a lot quicker this year and staying upright would help.
I will have out and back routes in my Garmin, but will have speed, average speed, distance showing most of the time. It will still beep if I go off course. I won't take my phone, but might take a small camera that could be charged via the USB port if needed.