Originally Posted by Bob Pringle
Right! But the longer arm(s) carry the TENSIONER pulley, which has practically nothing to do with shifting the chain laterally on the cassette cogs. It's the GUIDE pulley that does the shifting, and it's just as close to the cassette in a long cage derailleur as the guide pulley in a short cage derailleur.
Not trying to be a wise-(guy) here, but I don't understand the reasoning.
Because the OP said the shifting was OK but the chain was off to the side of the outer pulley.