The available trunk space with the top down on the XLR-v is considerably more ample than I initially feared from review comments on the XLR, and internet chatter. Of course, having driven Triumph Spitfires in the earliest years of my car-buying life, and having has a late C4 Corvette convertible, the 4 cubic feet remaining with the top down seems generous considering the huge space you get with the top up, and the benefits of a retractable hard-top to boot.
6-8 grocery bags seems about right. I have no trouble with a large briefcase + overnight bag + incidentals or 2 overnight bags plus a few incidentals if traveling with my wife, top down. Also the cubby hole in the rear right corner below floor level holds a surprising amount of cleaning gear, etc.
Top up, who would need more space, or more useful space, in a two-seater than what the XLR offers. It's nearly a hatchback! Sure, with a ragtop, there'd be more trunk space, but the character of the car would be nothing comparable. So, on balance, and knowing the car's Corvette platform basis, I think GM got the stowage compromises right.