Chad, in the fast passages should you use first valve, and nobody will notice. If slower, the 1-3 d will sound in tune. When I was a C trumpet-learning student, I was able to play Barnum and Bailey's Favorite at tempo with a bunch of cross fingerings plus transpositions on my C--it really helped train my third finger! My suggestion wuold be to stay with your horn and start doing the 1-3 thing, and with practice it will not be slower. Your body will learn! Have fun, and I bet your cornet sounds great!