There's a new company called Wavetrac that will be releasing torque sensing LSD units for the 996/986 6-speed in January. Their system is unique in that it has the ability to provide locking when one wheel is completely unloaded, which normal torque sensing LSDs can't do. It also claims to provide longer life and more gradual locking than competitors. On top of all that, it's cheaper than Guard by quite a bit. You might want to inquire with them, maybe if they get enough interest, they'll develop one for the 5-speed.
http://www.wavetrac.net/