Using Sigma 18-35 with Sigma 2X Teleconverter?

Has anyone ever used a Sigma 2X, or any Teleconverter with a lens like the Sigma 18-35?

I love the sharpness, 1.8 Aperture and look of the sigma lens, but would like a little extra reach.

I've never used a Teleconverter so i'm not sure if using a teleconverter will significantly reduce the image quality (I know it ups the iris a few stops.)

Depending on the Teel-convergdr, it might soften the image a little, in addition to to 2 stops, neither of which would “hurt” the Sigma, might even help it!
Cheers