I believe the Sigma 105mm (both the EX and DG versions) are not compatible with the Sigma TC 1.4x and 2.0x. I used to have the EX version and in the info sheet that came with the lens, it cautions against the use of this lens with the Sigma TCs, as the rear element will clash with the TCs, and either one or both will be damaged.
Hence the 105mm cannot be used with Sigma TCs. It was for this reason that I bought the Kenko Pro 300 TC for use with the Sigma. The Kenko's design is different and has no problem with the 105mm.
AF is really slow when the TC is attached though... ;p
The Sigma TC compatibility chart can be found here. Note that the chart is not updated for the latest 150mm which can be used with both TCs.