Dave,
I've got the Sigma 70-200mm and the the Sigma x2 TC, if you would like to see some of my photo's I took while at the WTCC rounds at Brands Hatch then I'm more than happy to post some tonight for you to look at.
For me I'm quite happy with the results I'm getting from this combination and it also saves me from having to carry the Sigma 80-400mm lens as thats one big lump of a lens

Bristol camera's are selling their's for £139 plus p&p
Sigma Lens 18-50mm 28-135mm 55-200mm 28-200mm 70-300mm and more
Hope this helps
