Certainly, for a larger, heavier car like the Subaru a bigger rim is mandatory. My WRX needs 16" wheels minimum to clear the stock brake calipers. And today, there are far more tire choices in 15" and larger wheel sizes. Personally, I prefer to stick with 13" wheels mostly because of the lighter weight on the wheel/tire assembly, and the resultant reduction in rotating inertia (less hp soaked up through the driveline). Acceleration will be better, and the reduction in unsprung weight will allow the suspension to better conform to road irregularities and improve ride quality. Mostly though, larger wheels and lower profile tires are simply in fashion, regardless of the loss or gain in performance from them.
If you want to go up in wheel size, I suggest staying away from 14" (hard to find) and going with 15" wheels. You could go with 16" wheels sure, but they're gonna cost more, weigh more, and if you race.... racing tires in 16" sizes are harder to come by and more expensive. 15" wheels will allow pretty darn big brakes on an Opel if you decide to go that route.
Bob