Agree 15 wrx and sti should be the same. To be clear I'm not saying it won't fit just recommending that one measures for themselves.
The issue on the '15 is there is less room from the hub flange to the inner quarter lip. I read this on several sources one source in particular said that hub to fender on 09-14 was 110mm and 15 is now 70mm. I don't remember the links as I did my wheel shopping last fall. I didnt measure a previous gen but my measurements and google image searches seem to confirm that there is less outboard room on the '15.
Check for yourself, stand a level vertically plumb against your fender/quarter (put a piece of tape on the level so that it doesn't scratch the paint) and then measure the distance inward to the wheel face (near the tire bead. Note the 15 stockers don't have a lip for weights) you will want to measure the top of the wheel since that's where the contact will take place and there is a surprising amount of stock camber in the rear. (The bottom of the wheel will be closer to the vertically plumb level than the top)
Ok so now you effectively have the distance for the outer wheel arch to the wheel face next you need to subtract for the inner wheel arch lip (the flange where the outer and inner are spot welded together). Measure the width of the flange, I recall it is nice and small but rolling will still offer some more room. I was not willing to roll, on a winter driven car, it's hard enough to keep rust at bay, without creating more tight little crevices.
Ok so the stock wheels are 17x8 +55. Most wheels are actually 1" wider than the tire mounting width. So an 8" wheel is actually 9" ... a 9.5" is 10.5" and so on. This has proved true of the wheels I have had over the years. So the stock wheel should have an approximate front spacing of 2.34" and a backspace of 6.66".
A 9.5" +38" wheel will have a front spacing of 3.76" and a back spacing of 6.74".
The '15 sti wheel is 8.5" +55 so ... F 2.59" R 6.91"
You can see that the back spacing will be fine as it is less than the sti stock wheel. In fact if your goal is to fit the maximum tire patch while keeping the offset as close to stock as possible I would push back more. The most back spacing i have seen is OZ 9" +55 which works out to R 7.16". With coilovers you could go further inward.
The front spacing is where opinion comes in.....
As you can see from the numbers above that a 9.5" +38 wheel will project 1.42" further from the hub than the stock 15 wrx wheel. I can't double check right now, but that is way more poke than I thought I could run without rolling the quarters and stretching tires.
As stated before "fit" is highly subjective and for the price of quality wheels these days it's important to get exactly what you want (look, contact patch, brake clearance). Using this method you can spec a F/R spacing calculate the offset and width and then go wheel shopping. I have found that excel is very helpful for narrowing your search.
(I know that this is common sense but I thought it might possibly help some wsc'er)