There was some discussion of air flow through the engine room and effects of trunk in and trunk out a few months back on the DeTomaso list. If you want to dig through the archives or post the question to the list, start with:
http://realbig.com/mailman/listinfo/detomaso I think the end result was the air conditioning and top speed were better with the trunk in. Hot air is forced to go out the opening in the decklid. With the trunk out there is a lot of turbulence where the trunk would be and hot air fighting your condenser fan.