I have gone with a setup with 87.9 GI top gear and was fine. I probably could go lower if I had to, but see no reason why I'd want to.
I am not sure but I would assume so, though it is a minor amount. I have bikes with a traditional derailleur system, the fairly well known Rohloff, and the aforementioned Pinion P1.18. To me the
perceived efficiency is traditional, Pinion, Rohloff. I am sure someone more scientifically minded than me has tested each to determine the efficiency.
If you are doing a ride primarily on paved surfaces, then I would suggest using the traditional system. It is definitely proven for decades, easy to maintain (though much more maintenance is required), and much less expense initially. It
might be best on
dry fairly
groomed unpaved surfaces (crushed gravel on rail trails) but then you have even more maintenance. For any wet or or ungroomed surfaces, I like the easy maintenance and no derailleur of an internal geared system as mud can't gum up the derailleur and snap it off or have the chain jump off on bumpy roads. Plus the maintenance if an internal geared system is fairly maintenance free, especially when paired with a belt drive.
For instance, I am planning to ride from Prudhoe Bay to near Mexico this summer. Due to the unpaved and muddy conditions, I will definitely take the Pinion/belt bike. I will start off with a freshly oiled gearbox and the only maintenance I will have to do is wash the dirt/mud off the belt with a hose (or more likely my hand by a stream) periodically if it gets bad. Otherwise, no maintenance which a great for a lazy guy like me. When I get back home, I will re-oil the system which only takes about 15 minutes and is very easy to do. IMO, the Pinion is easier to maintain than the Rohloff, especially dealing with cable changes.
That said, and to bring this back to the topic of the post, the gear range to me is too wide on both Pinion and Rohloff. The manufacturers had to come up with some range but really do wish it was smaller/tighter as I almost never use the very highest gear. I really miss the old days when you could pull a freewheel apart and put on whatever cogs you wanted to get you the gear range you wanted. Even then, my lowest was a 15t (maybe a 16t, can't remember) with a 39t chain ring as I never wanted a high gear because back then I had a 110-115 cadence.