Gill
I use a Sigma 70 - 200 f 2.8 (which is a nice bit of glass) and have the sigma 2x converter, which makes it a 140 - 400 f 5.6.
I am always disappointed with the results when using the converter.
Part of the problem is that in order to shoot handheld you need to increase the shutterspeed to 1/640 or 1/800.
Even in good light it means "jacking" the ISO up and my Canon D40 has poor high ISO performance.
Below are some images from yesterdays shoot. The light was fantastic so I got away with keeping it to ISO 400.
The first 2 are shot using only the 70 - 200 at 200 mm showing the original size and a crop.
The second 2 are actually at 318 mm and again the original size and a crop to approx the same size as 1.
The first crop shows more noise in the blue (which would be easy to fix) but is substantially sharper then the second one.
I am now seriously thinking of selling the converter. (any one want to make me an offer??

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If I started again I would probably look at the Sigma 50 - 500 which Bert uses or the Canon 100 - 400 L
Theo