A 270 has a "guide number" of 27 meters. That's based on ISO 100 and an f-stop of f/1. While you're not shooting at f/1, the point of using that as the base in the guide-number is that it makes the math VERY easy -- just divide the guide number by the f-stop you are actually using.
E.g. if you're shooting at f/5.6 then you'd simply divide the guide number value by 5.6. That would give you adequate coverage out to 4.8 meters (just under 16' -- btw, Canon claims the flash is good to 16.4') HOWEVER... this assumes you are using the 270 with it's most concentrated beam of light (meaning you pull the reflector forward to the "tele" position.) In the wide position it appears to lose about 1/2 stop.
I did a quick test with my 270EX (I carry one of these with my G1 X) by putting it on my 5D III body. When testing a "white wall" exposure (actually I laid on the floor and took a photo of a white ceiling) I get noticeable vignetting in the corners at 24mm (on a full frame camera). If you use a crop-frame camera you'd probably have a _very_ tiny amount of vignetting in the corners -- so little that I wouldn't mess with it.
The "problem" with using a shoot-through diffuser is that they all eat light. I'm going to go out on a limb here (meaning I'm not looking up specs nor testing a diffuser) and suggest that a sto-fen diffuser is probably eating about 1 stop worth of light (so that's halving your power or reducing your effective range.
Also note that these distances were based on ISO 100. Each time you double the ISO you get to multiple your shooting distance by 1.4.
Tim Campbell
5D III, 5D IV, 60Da
