Am trying to decide between these two lenses to use on a 30D. If reach wasn't an issue, of course I would go for the faster 70-200, which by all accounts is a great lens. However, probably the majority of the time I will be shooting surfing at 400mm. I figure the 70-200 + 2x converter will give me a maximum aperture and reach as the 400. What disadvantages would there be in using the 70-200 + 2x tcon combination over the 100-400. I've heard that I will lose autofocus ability, image sharpness/quality etc. Is this true? Anything else to consider?