The fact that you're using a 20 amp rated outlet will make no difference on a 15 amp circuit. You could use a 30 amp rated or 50 amp rated (I'm stretching here) and it would make no difference. The difference is in the quality of the outlet. You could take the same design in a 15 amp rated outlet and achieve the same result as your 20 amp. What matter is the circuit breaker in the box. You're only going to be able to draw 15 amps from that circuit no matter what the outlet rating. I just wanted to make sure that was clear.
Upgrading power system
Since I already have components and speakers that I love, I try to make an improvement every year with something or another. Last year I bought Magnarisers for My Maggie 3.7's. Nice improvement. Immediate difference. The year before was power cables and conditioner, all Shunyata. And I always wondered why there was little if any improvement. A couple audiophiles said to replace the wall outlet. So I thought about it for years before buying a JPS labs audiophile hospital grade outlet. After removing the original outlet, I compared them. No comparison at all, the JPS weighed almost twice as much and the metal was better quality as well. The JPS was 2o amps and the original was 15. Would I hear the difference? It took all of one minute. Best $150 I ever spent. I was skeptical until I heard the difference, mostly clearer and more pronounced highs and well defined low end. Anybody else have this experience?