I was just able to taste my single-hopped Amarillo IPA last weekend. I think it tastes very grape-fruity. That being said, it is very good. I feel that it lack any backbone though. I would go with at least 2 oz of magnum since it's an 11 gallon batch. From what I understand, you have 10 ounces of Amarillo? To try and stay in check with what you are doing, I would go 2 oz at 15 minutes, bump the 5 to flameout and change it to 4 oz, and dry hop 2 oz in each corny. If anything, consider upping your bittering hops. But whatever you do, it will be very good! Good luck and let us know how it turns out!