I was testing the Tamron TC 1.4x (non-reporting) on my Canon 40D and 400 5.6L lens. Gitzo 6x carbon-fibre tripod used and Canon 580ex II flash. The results were pretty good. This crop is almost ful-frame top to bottom and cropped on sides.

Non-centre AF point is best. Centre AF point with the 40D and a Tamron TC 1.4x does not work well at all due to the extra sensitivie and accurate centre AF point on the 40D.