A few suggestions
I hate the stuff normally, but wonder wheels is just about the strongest stuff you can buy and is a fairly strong acid. Use this coupled with a brass wire brush. Don't use a steel wire brush as it will seriously damage the paint and alloy, but a brass brush should be ok. Work in the direction of the wheel as opposed to across as any scratches that do develop will be less visible. Make sure you wear some eye protection as you will splash stuff everywhere.
It also might be worth getting some clutch/brake cleaner which is about £6 for a can.
You could also try oven cleaner, but this stuff is viscous and so you don't want it on the alloy for more than about 20 seconds.
E90 330i SE Auto Graphite Grey Metalic