Back in the day, we used the Hone-O-Drive (attached directly to the differential) in the '70 GT500 w/ 4.88's, and it was great for the dual purpose function, but, it also proved to be the weak link in the drive train when coupled to the 461 cu. in. Tunnel Port w/ the 12' x 29" Hoosier dirt tires.
Therefore, if attending a track event we would swap-out the differential for a non-Hone equipped unit. I suspect that this may also have been a concern with the proposed 2-speed unit, as the compact dimensional requirements would possibly have compromised strength requirements. And of coarse, costs & marketability considerations.
Scott.