The worst cable friction problems I've encountered weren't the ends of the housings but instead were cases where stainless inner wire was used in the stainless housings and/or run across a stainless cable guide under the bb shell (Trek's big mistake on early OCLV frames).
Stainless steel microscopically cold-welds to itself, producing the highest coefficient of static friction imaginable!
The old stainless housings also are quite elastic in compression, which only
further degrades friction or indexed shifting.
Choice of lubrication will be optimal with plastic-lined housings with a non-petroleum Silicone/PTFE grease applied. The very best housings from Shimano and Yokozuna are pre-lubricated along their entire length with low-viscosity Silicone/PTFE grease. SRAM's Jonnisnot cable lubricant is similarly a Silicone/PTFE formulation, but made for localized cable path guides found in GripShift and DoubleTap shifters. But it is also really excellent applied lightly to the entire inner wire, especially
after perhaps having scrubbed out the old housings using
PTFE/solvent-based Dri-Lube spray, a bent-up cable and compressed air. I still use this (my old self-taught method) to refurb
vintage bike's original housings because the SRAM grease is
so very slippery inside of a clean, plastic-lined housing.
One more thing about silicone-based lubes is that they do not evaporate, oxidize, thicken or permeate the housing's plastic liner. So the cable housings that you tune up using silicone grease should still feel tuned up thirty years later after sitting all that time in your garage.
Lastly, after refurbing my dumpster-sourced Pro-Tour in December, and after having worked on and ridden it since then, I finally realized that to get the smoothest shifting I was after using the bar-cons, that in addition to my other cabling efforts I had to put a piece of plastic tubing into the groove of the clamped-on cable guide at the bottom bracket. I used an inch of housing liner tubing and after fitting up the cables I put a dab of epoxy glue in the groove to keep the housing liner segment from sliding out. After doing this and completely removing the Suntour B-tension adjustment screw, the shifting is quite good enough for sporting use in our rolling foothills environment!
Bar end shifters will always benefit (per Gugie's advice above) from any extra bit of setup care especially because the cable to the rear derailer is so very long. Friction forces effectively are multiplied by the long cable's (and housing's) elasticity to produce high error between shifter and derailer movements (i.e. vague shifting response) unless real effort is made to improve the housings and overall cable path.
(cable + housing elasticity) X (bi-directional friction force along the cable's path) = (hysteresis or motion error between shifter and derailer)
