Hi Gman,
IDPA's Enhanced Service Pistol Division (ESP) is for any 9mm and over. This includes .45's.
IDPA's Custom Defensive Pistol Division (CDP) is for any gun in .45acp only. Any other .45 (such as the .45 GAP) is not legal for CDP.
In the case of your SIG, after looking at the specs, this gun would be legal in ESP and CDP.
Since the maximum magazine capacity in ESP is 10 rounds, and the maximum magazine capacity in CDP is 8 rounds, you may find yourself better off in CDP as you will not be faced with a 2 round deficit to your fellow competitors. Also, since ESP is based as the 9mm as the minimum caliber, the power factor (Bullet weight multiplied by velocity) is lower for ESP than CDP, so you will be shooting the stiffer recoiling .45acp against the 9mm & .38 Super shooters. While usually it is the shooter and not the gun, you might find it an easier path to try IDPA in CDP with that gun initially.
Ted Murphy
IDPA Area Coordinator for Eastern Pennsylvania/New Jersey