have a 2011 subaru impreza 5 door sport package with 16 inch stock rims

Long as you put the right tire on it. It would be a lower profile tire to match the same diameter of the 16" tire.

205 /50 R17 is the size tire you want.

Yup; but even larger sizes fit fine. I just put 255/55-17 on a buddy's new Imp with crappy steel 16's to improve handling AND especially to get added ground clearance for driveway dip. Given your 2011's gutsy 2.5i motor, 215/50R17 would be my favorite size (as used in modern Legacy Limiteds), as the 205/50 WILL feel great in the twisties but pretty stiff over potholes. 215 will give you a bit more bite, 1/4" road clearance, and slightly better cushioning too. But 225 is overkill for the Imp except for really aggressive semi-race use.

Subaru keeps using 205 and 225, For the wimpy 2012+ 2.0i motor the 205 is fine; 225 is overkill for the 2.5i (except the taller OB).I've always thought that 215 width tread is the best compromise for the 2.5i. Just make sure you get the correct aspect ratio (in this case 50) for your app. 215/55-17 is rare, but will lift the car an inch, and offer nice pothole forgiveness. 215/50 will look a tiny bit bigger than stock, but feel better in the twisties and potholes than 205/50. Got it?