Yeah, that's a mistake I definitely don't want to make. I never imagined internal routing to be such a pain in the ass. The 'sheath' is very similar in thickness to a drinking straw (thin plastic), and it runs the full length from the cable input on the downtube, to below the bottom bracket. There's a second sheath for the RD that continues through the chain stay and comes out near the drop out.

The sheaths aren't attached to anything internally, and they just sort of rattle around. I just can't think of any reason that they would need to stay on the cables once they're routed, other than maybe to prevent internal rattling? The more I think this through, the more I've convinced myself that removing them is the way to go, but just want to make sure I'm not missing something obvious.

Thanks again for all of the responses.