Sigma 70-300mm and TC?

Am I doomed with this lens (Sigma 70-300mm and Canon 400D) and not being able to use a teleconverter? I understand I can "try" something like a Tamron TC, but chances are it's not going to give me good results.

I cannot buy a new lens until summer, so I'm looking into a TC for the time being. Has anyone been able to use a TC (and which one? is it true the focus gets soft with a 2x?) with any amount of success with this same lens?

I'm baffled and feeling doomed.
Any glimmers of hope out there?
-Kaz

You won't get AF without playing tricks on the camera, and then at the risk of hunting for focus. Neither Canon's nor Sigma's will physically fit; Tamron does make a good quality 2xTC as those things go, but 2x TCs aren't noted for providing good contrast, sharpness, or saturation on primes, let alone zooms. If you need a bit more reach, consider the 1.4x TCs, which are much less deleterious to your IQ. You still won't get AF though.

so a Tamron 1.4x would still be worth a try for this lens, even without AF?
Let me ask this... since I've never seen a TC hands on.... when in manual, would you focus the TC first and then your lens, or how does that work?
-Thanks,
-Kaz

The TC is just a couple of additional elements added to your lens. You use the lens normally, except instead of being a 70-300 f4-5.6 it's now a 98-420 f/5.6-8 and won't AF. The viewfinder will be a bunch darker, too.

If you really need, right away, that extra reach, and can't afford to get a longer lens now, or wait until you have the funds, a TC's the only solution (well, there's always cropping and upsizing your pictures). Not the best solution, but the only one meeting your needs at this time.

Thanks for the help, I appreciate your time and knowledge!
-Kaz