I shot a 3.5" gun at my last IDPA Sanctioned Match (state championship), and along with the one-round capacity deficit, it is extremely difficult to be competitive with full-sized guns. The short guns can be plenty accurate, but they are much harder to shoot.
The top level of IPSC/USPSA competitors have been on-again/off-again with the 6" guns. In the early '80s, a Hoag longslide .45 was the way to go, then nobody shot 6" guns for 20+ years, but they made a come-back for a while, about five years ago. It's a fad.