I've got the "30" (the biggest one") and it's by no means too big. It's only the size of a small rucksack and is still quite easy to sling around to the front, despite being wider than the "20".
It will hold 4 extra lenses, in the way that I have the bag configured and has really useful side pockets and a very big top pocket for quick access to whatever you decided to put in there.
I think that it's an excellent bag, although I've not tried any similar size bags from other manufactureres, so I can't offer any comparisons :shrug:.
All I would say is that if you
think that you might need the lager bag, then
get it! I've read so many reviews on Amazon etc., where people have said things like, "I used to have a Slingshot 200 (for example), but I upgraded to the 300". That must have cost them something

.
In answer to your questions, I can carry every lens in my "Camera Bag" profile (go have a look) and a D700 with the 70-300mm VR on, plus loads of other gadgets in it

.