I just bought Sigma Macro 180mm F2.8 EX DG OS HSM and I noticed that Canon TC's cannot be used with it. Looks that Sigma has just added additional (too long) collar behind the rear lens. That collar will prevent use of Canon TC's. If collar would be just 1-2 mm shorter Canon TC's could be used.
Has anyone tried to cut that collar little bit shorter? I removed bayonet of the Sigma but I couldn't figure out how that collar is mounted. Maybe it has only threads that will keep it around the rear lens mount. That collar seems to be made of plastic so I believe that it is not the actual mount for the rear lens.
Surely modification can void warranty, but lets not care about that