Hypothetical - could a dealer even tell if a 30 grade was used instead of a 20 grade? Just curious to what extent they would go if say you had a warranty issue and didn't use a 20 grade. I suppose they'd like to see records of oil changes to start.
You're talking about a HT/HS from 2.6-3.0 for most PCMO's today. Pretty insignificant. If they ran viscosity tests you're looking at mid 8's to 10cSt. The modern fuel economy 5w30's could potentially give you a tad more protection in theory.
I didn't realize the current WRX called for a 0w20 grade. I'd have no problems with a 20 grade but would highly consider a 30.