IIRC such cable guides were specifically forbidden by most manufacturers for indexed gear installations of any kind. They can usually be fitted with a length of plastic liner of some kind, but I have found that unless the liner itself is fitted with drain holes, it tends to fill up with crud and if there is enough road salt, even stainless steel cables can start to corrode.
BTW when fitting a new cable to any potentially worn underbracket cable guide, a new cable can be fractionally wider than the old one, and can almost immediately be much draggier than it should be, because it (literally) jams in the groove worn by the old cable. In any case more tension begats more friction, so if the shift is baulky for any reason, a disproportionate increase in cable tension may be required because of the increased friction.
Currently Campag recommend that you measure the force required at the shifter end of the cable during installation, e.g. in order that
a) the RD can overcome the cable friction on upshifts
b) the RD spring tension is correct for the shifters (not every campag RD is compatible with every shifter, even if the cable pulls are the same)
c) that the cable friction isn't so high that downshifts routinely overload the mechanism.
Anyway my Mirage levers have had worn hoods (worn smooth twice over), worn internals, worn brake lever bushings, you name it, but the steel/plastic paddles are showing no signs of distress. But then they probably haven't been used with super-draggy cables either.
cheers