Yes, I know there’s a tiny bit more friction…
There’s less friction in a single outer. Friction in a separated system comes from the cable ferrules/cable stops, and that’s even before they get a build-up of crud in them.
I disagree but am in no position to prove it. From experience full outer never works as well and when it does start to get dirt in which it will from the rear mech end it’s worse and you don’t want to do anything about it as it cost more and should be better. YVMV
Shimano cable and sealed ferrules and a no brand outer although I’m going to try and source shimano outer in a big roll next time also. All bought in bulk.
Typically getting 400 miles of all weather riding out of them but that’s been due to a few damaged mechs and hangers, shifter changes in the last couple of years. I’d probably get double that with no issues. I tend to put new on every spring once winter has buggered off.