I've got a bike with almost exactly this setup. I
think mine is 53-42 and 13-26. Maybe it could do 28t but there's not much wrap, it also only does 28 links. You also can't go 8+ speeds with the 7-speed version. It won't swing over the whole width. I recommend a different RD.
Bike Forums - View Single Post - RX100 Rear Derailleur
VeloBase.com - Component: Shimano RD-A550, RX100