A lot of Subaru converters are using copper pipe to re-route coolant
lines inside the engine bay. Others are taking a more expensive route
($10/foot) and are using marine exhaust hose. Granted, this is a special
application. I'll be using copper when my subaru engine goes in in just
a few days.

None of these converters (that I know of) have experienced any problems
as long as their coolant is up to snuff.

But, if you van get original steel replacement, why not use them?