^^Actually I found running the stock tire 215/60/16 (26.1" tall) my speedo read 55mph when in reality I was doing 52mph(verified with 2 GPS units). Subaru speedo have a history of reading fast, every one I have owned has read about 3-5mph fast at 50mph actual speed.
Running 225/60/16 (26.6" tall) brought the speedo and actual speed close enough together to call it dead on.
Now running my 215/65/16 winterforce (27" tall) the actual speed is just a hair over the speedo by 2-3%
As far as the tallest tire in a 215 width I would say 27"(give or take) is what I would keep it too. My winterforce are close to rubbing at wheel lock under some suspension compression. I am almost sure the next series up would rub.
Granted I am dealing 16" tires. For 17" I believe that(27") equivalent would be a 215/60/17 using the tire calculator Bobby posted.