I understand there's a huge convenience factor for replacing with rubber hose, but it seem to me that copper tubing, especially if up-sized an extra 1/8", would present less frictional losses. That would allow moving the attachment point on the rudder arm, and thus reducing your turns lock-to-lock by a certain amount.
Or is this rather insignificant in most person's minds?
Agreed, the real value of metallic tubing is it does not expand, and so steering is more positive, less spongy. The longer the run, the more of an issue that becomes. I hate removing copper tubing if it's sound, if just the terminations are the problem, because copper tubing is essentially forever, and it has no expansion issues.
Steering manufacturers often spec maximum recommended distances for hose.
Thermoplastic tubing is easy to work with but it's not nearly as durable as armored hose.
Remember, the relief pressure for most manual hydraulic steering systems is 1,000 psi, so all the parts you use must be rated for that pressure, which means no cast bronze plumbing components, while most machined brass is rated for this pressure and higher.