Richard, I hear what you are saying about the different expansion rates but Past Parts don't seem to think it is a problem. From their own site:
Units are bored and re-sleeved back to their original bore size using stainless steel which means it won't corrode, a valuable advantage for vehicles stored for long periods. Even if a new part can be supplied, a re-sleeved unit may still be preferable, especially if the casting is aluminium, as it eliminates the common problem of bore pitting and corrosion.
Presumably there is still a potential problem if the chrome plated steel pistons corrode??