Ger's 2003 Porsche Boxster S

[TrumpyAl]

I agree, it would take a lot of treatments to move that kind of buildup, and you'd still have staining to remove anyway. A wheel brush like a Daytona Speedbrush is the best weapon in your arsenal against buildup of that magnitude forming in the first place - when coupled with a user friendly non-caustic wheel cleaner they make it so easy and pleasant to maintain the drum. A small brush for around the nuts and you're set.
Iron-X or the like I'd recommend before your periodic wheel wax/sealant. So every six months for me. (but we don't have salted roads to deal with down under)