You'd never go down in tread width as well as aspect ratio at the same time and hope to have the same outside diameter.
255-50R17'* are as close as you can get to the same outside diameter and offer a more substantial contact patch. If you wanted to plus size, those are what you should get.
For my winter tires I was thinking of down on the tread width and up on the aspect ratio. For the same outer diameter, 215-60R17 fit the bill, but they are alittle hard to come by.
With more cars coming with stock 17" rims, the tire selection should be catching up some time soon. Many tire manufacturers either don't offer +/- size tires or they are too expensive. Price will determine what I get as far as the size of the winter tires I'll buy in October.
Oh... I was pulling my comparison off of this, Tire Size Calculator