Paul
I actually bought a slingshot 100 for my 20D + 17-85 attached, 70-300 with future acquisition of sigma 105 macro planned.
I took it back and changed it 2 days later. It held all the equipment fine - the problem was that the zip for the opening flap is not near enough to the edge of the bag. This means that, when you swip the bag round to get the camera out, the camera grip is under the side of the bag and it makes it very difficult.
slingshop.jpg
Not sure if this would be a problem with the bigger 200 but my advice would be to take all you equipment to a stockist and see how you get on