As stated in an earlier post, the engines have different compression ratios. Also, GM even gives different engine codes for them. The Hummer engine is listed as a Vortec 6000 LQ4:
http://www.gm.com/automotive/gmpower...ec6000_lq4.htm
The Cadillac engine is listed as a Vortec HO 6000 LQ9:
http://www.gm.com/automotive/gmpower...tec6000_ho.htm
Klaus