what with the price increases the 100-400mm (and also one or two of the sigma superzooms) have been looking more attractive to my eye - but I have my 70-200mm and a 300mm f2.8 would complement that well. A 500mm after than and I think I would be sorted (so thats - what 10 years or more to go

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The 300 f/2.8 is really a nice lens and is sort of on the radar scope for indoor event sports (an area I'm looking to get into as a source of income

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I've watched your progress over the last year or two and I think I have a feel of your wants and needs being balanced against costs.
If you like to do wildlife (and I don't mean at the zoo but in the wild) your 70-200 with TC's will do the job (even with a 2x) but your likely to miss shots trading out TC's. This isn't going to happen with the 100-400.
I wouldn't advocate you selling the 70-200 as this is really a fantastic and absolutely tack sharp lens. It's uses are wide rangeing and is a hell of a portrait lens. I would however get the 100-400 before the 300 f/2.8.
Reasons? Well, first off, It's a hell of a value... About the most bang for your euro and pretty easy to resell if you so decide. It also is more quickly in reach (money wise) than the 300.
It will help you decide what you really want and need in a super-tele prime. You may want to skip the 300 and go right for the 500.
It takes fantastic photos and with todays high ISO performance the slow aperture is not that hindering.
I would also recommend upgrading in body to the likes of a 40D also before buying a fast prime.
Just my thoughts.
Last f/2.8 is really nice to have but it's not magic. with its shallow DOP at 2.8 getting the shot is not a piece of cake.