I am also short (I'm 5'4" with extra short legs) and I have a hard time with 8" shorts as they get me behind the knees after awhile. I use the PI Ultrasensors for winter rides where I am wearing leg warmers, but they are too long for comfort in the summer.

Have you tried the PI Attack Short? It's identical to the Ultrasensor, but it's a 6" inseam. Same chamois and material... usually it's like $5 or $10 cheaper than the ultrasensor as well. They aren't as easy to find as the Ultrasensor, but some places do carry them. I found that sizing was identical between the two as well.