I'm on a personal campaign to eliminate 4 millimeter shift cable housing, especially with cheesy plastic ferrules on the ends.
Installing 4 mm housing is like purposely having your arteries surgically narrowed. Actually it's more like having your tendon sheaths constricted, but the artery image is more vivid.
Modern super-indexed sifting systems, upwards of eight speeds and completely dependent on perfect functioning of a proprietary lever assembly, rely on smooth cable movement. Yet time and again I find original equipment cables on new bikes burly enough to moor a ship, running through these dinky, skinny housings.
True, most mechanics just pass this along to the rider. In all likelihood, professional teams use 4 mil housing, too. Of course they have professional mechanics to go over the bikes daily during important events. And most bikes that go through our shop without the housings changed work well enough for months. But 5 millimeter housings help a lot when a customer brings in a bike with balky shifting. Metal ferrules resist splitting better than plastic and also hold up much better to the pressure of the linear inner wires in the stiff housing. You'd be hard pressed to feel the difference in weight between a 4 mil set of housings and a 5 mil set.
Cable routing and housing length make a big difference too. I'm really glad manufacturers seem to be backing away from putting shift cable stops right on the head tube. That did not solve the problem of interference between the front brake cable and whichever shift cable was routed past it. The housing actually arcs more cleanly when the cable stop is mounted down the down tube a little way. Miraculously, the industry seems to have picked up on that.
Crossing the shift cables in front of the head tube eliminates a lot of head tube chafe. This is critical with carbon fiber frames, because they are so much more vulnerable to abrasion damage. Unfortunately, just because the cable housing stops are on the sides of the down tube does not mean you can cross the cables. The stops need to be far enough down the circumference of the tube to allow the cables to cross back again under the down tube to reach their respective derailleurs.
Some of this is old news. Cable crossing has been done for a long time. Some bikes even come from the factory that way. I repeat it in case anyone was absent that day or just came in.