The Canon TC's do not work on the Sigma. Expect to use only Canon lenses w/ Canon TC's. Even then, Canon TC's are not universal to all Canon lenses.
Kenko 1.4X PRO 300 Teleconverter DGX for Canon EOS Digital SLRs - I use this and you will still get AF w/ some caveats. Older versions of this Kenko TC require a pin tape (just Google it) for AF to work but the one I received in 2010 did not require a pin tape - AF worked out of the box. I can vouch that this lens and TC work together on a 450D/XSi but always check Kenko's site for the lens compatibility chart if you decide to use it on another lens (not sure if Amazon has the chart posted on the product page).
The 1.4x Kenko brings the benefit of 210-700mm at the cost of two stops, slower AF/additional seek time, and the occasional inability to get an AF-lock. You get some pictures you would otherwise miss and lose some pictures you might have otherwise had. This Kenko also works with numerous other lenses - Canon and Sigma. Probably Tamron but that's just a guess. So there is definitely value to the Kenko TC beyond the 150-500 Sig.
Important Notes: - All TC's cause a loss of light and and can be expected to bring some loss of image quality based on the quality/magnification of the TC (just a bit of softness in this case). This Kenko will lose you around two stops but grant 210mm-700. The impact on picture quality is nominal/immaterial to me but each photog is different. - Do not expect AF to work 100%. The AF will seek especially w/ low contrast subjects where the AF is targetted (darker object on dark bg, lighter object on light bg). Don't even try it in poor lighting. This is just the nature of light loss from the TC and a f/5.6 lens.
**Anytime you use a TC, be sure the appropriate lens is listed in the applicable lens compatibility chart. Using incompatible cameras/lenses/TC's could result in permanent damage - smashed mirrors, scratched lenses, etc.**
The canon converters will only work with canons L lenses from everything I've heard. You can use the sigma converters, which will work on this lens I believe or if you buy the kenko teleconverters they work with just about any lens and cost a little bit less than either canon or sigma.
Here's the problem if you use a teleconverter on the Sigma lens you probably will lose auto focus. You already lose two stops of light. The teleconverter will work with autofocus if the fstop is 5.6 or smaller. If you are you are going to go this route you will need a tripod, have to manually focus. and it will have to be on a bright day or you will need a long shutter for the decreased light. I imagine a monopod might work but being that you are going to lose so much light you will have to use a long shutter to compensate for a potential large aperture effect. I will say I haven't done this but I have researched the Canon and Kenko teleconverter. The teleconverter effect will be the following: 1.4 loses 1 stop of light, the 2.0 converter loses two stops of light and the 3.0 on the Kenko loses 3 stops of light. The auto-focus works at 5.6. Even at a fstop of 5 which is between stops. A quarter or a third of a stop gives you a 5.6 so the fstop is going to be greater than 5.6 with a teleconverter of 1.4. According to Kenko their teleconverters lose auto focus at a fstop greater than 5.6. Let's say that you able to get it to work your auto-focus is going to be very slow. Focusing on things is going to take a long time if on auto focus. OK I haven't used a teleconverter but according to Kenko this application wouldn't work very well.
This lens works great at F8 or more. Hand-holding is possible if you have good technique. I have tried using TC's on this lens, but to be honest, you are better off not bothering. If you want a bit more reach, get closer to your subject and use an extension tube. For some sample images, check out the recent section of my website www.nicksaundersnature.com