I did just that with a short cage Ultegra rear der for a couple of years on the Major Jake when I used to find myself out with the mountain bikers on the lunch rides. 48/38 up front...12/32 on the rear. No problems whatsoever.
If you already have the ultegra and you know you want to run that cassette, you have nothing to lose by giving it a shot (make sure you set it up well).