Well, you only should use platinums if it's a PITA to change plugs, or are lazy and don't like changing them. You are better off using regular copper plugs as opposed to platinums. I've always used NGK's...and they work great for my RS.
I'm using the Plat +4's, and apart from my not tightening one of them down, they seem to be fine. Note that if, after you install them, you find your gas mileage going in the toilet, check that they are all snug. I had one loose, apparently losing some compression on the #4 cylinder.
I haven't really noticed any performance difference, though they do seem to idle a touch smoother.
I'm still running a set, a smoother idle and better off idle acceleration was nice. They made a noticable difference for a while [10 months?]. At this very moment I can't tell the difference anymore maybe I'm used to them or the gains have deminished.
Check your subaru manual its in their.
Normally the gap from Ngk is good enough within specs
The wider the gap the more power,but to wide a gap your ignition system has to work harder. The smaller the gap the better starts you get. You need a balance for your car.