I'd be willing to conduct a subwoofer measurement day but it would require built units actually be sent to me in the Spring/Summer because all the measurements have to be done in the parking lot.
BTW: The only thing about the Tempest & SDX that is the same, is the fact that they are both 15" drivers with an XBL^2 motor. The motor technology only modifies the BL curve. It won't have any impact on the FR of the transducer, either good or bad. It will change high-level operation. When built right, it should lower non-linear distortion due to motor BL changes and all the T/S parameters that change with BL become more stable since the BL isn't changing with stroke. Either way... just because two drivers use an overhung or underhung motor, people don't lump them all together in the same category. I'm not sure why XBL^2 drivers get lumped either.... but I digress.