Yeah, think the Sigma suffers from the same problems as the Canon, namely that the lens assembly protrudes from the TC and so can foul certain lenses. The Kenko doesn't have this disadvantage and so I use it on all my lenses (though yet to try it with the MPE 65mm!!)
The "tape trick" works very well and I use it regularly - I've even stacked 2 x 1.4x TCs and still maintained AF (and, more importantly, achieved halfway decent pics - near as good as any without any TCs)! Yes, you lose 1 stop per 1.4x TC and yes, you need pretty good light (as the AF depends on contrast differences for accuracy) and yes, you lose some IQ but when needs must...........
Here's more info on TCs and how to perform the "tape trick"
Kenko 2x Teleplus Pro 300 Teleconverter
BTW, think the latest TC is the DGX version of the Kenko Pro300 1.4x