None of my Benzes has needed a waterpump in a vary long time. though I did install a new GMB- Thailand unit in my '96 Camry a couple years ago.
So long as they don't come off the shaft, or the cooling system hasn't been grossly neglected, I don't think there's a significant difference in function of the cast vs stamped-steel impellers. But I'm not a fan of the ever increasing use of plastic for certain items that used to be metal. At the BMW shop I used to work at, there were some failures of the original plastic BMW pump impellers, and all the replacements had welded steel impellers. Also recall replacing the cracked original plastic thermostat housing on a '90s Mercedes with a cast aluminum one.
Today I replaced my 240D thermostat after seeing the temp guage has been staying at the bottom for quite awhile. Got lucky that the three tiny bolts came out easily. The replacement Wahler thermostat was marked 'Germany', though I've seen some replacement Wahlers marked 'China'.

Happy Motoring, Mark

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DrDKW