I never understood why they make the bonneville gxp less hp than the grand prix gxp. I thought the bonneville was an upper level car than the grand prix.

by the time the GP GXP came along I believe the Bonneville had been dropped or on the way out. The Bonne GXP is a 300hp engine while the GP-GXP was 303, so very little difference. The engine choices were more about the tooling that already existed for the respective platforms as opposed to hp ratings. This is still evident by the Buick Lucerne which still uses the NorthStar and the Lacrosse Super that packs the LS3 v8.

I think the difference were due to PCM tuning and the tranny backing the motor. Given the extra weight of the Bonnie over the GP I'm sure torque management is running rampide in the Bonnie to prolong tranny life.