Yes, it'll hold the D2 bodies with bracket.
The 300 is slightly deeper than its smaller siblings thus handles the pro bodies a bit easier.
Six lenses is no problem depending, of course, upon their size.
Its a well designed bag, I got one a few months ago before they were formally released and I've run it through its paces. Works well for the average sized individual and bears weight much better than the smaller slings. If you're a tall shooter, you may find the bag too short for your torso. I'm 6'3' and I can't get the shoulder strap/waist belt relationship where I want it to actually have the belt take the load on the hips. Unfortunate as I really like the design for my climbing about in the canyons and such. I'm selling mine and going back to a Mini-Trekker.
For most people I'd give the 300 a solid 'excellent' rating.
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Holmes