The AW300 will not hold that, not by a long shot. Plus, it is insane to carry this much weight in a slingshot, it will kill your back. The Flipside AW400... maybe.
I've carried my Slingshot loaded with my D90 + grip, 24-70 2.8, 80-200 2.8 AF-D two ring, 2xSB700's, PW Flex TT5/TT1, extra batteries, battery charger, light meter, and I'm pretty sure there are a few more things I'm forgetting. I wouldn't want to climb Everest with all of that, but for normal day trips its comfortable. ANY bag with that much equipment will get tiresome to carry after a while.
ETA: In fact, just last weekend I carried my bag loaded with my D90+ grip, 24-70 2.8 G, 2 SB700's, and my entire Hasselblad 500cm set up complete with left hand grip! .
Excuse the iPhone photo, but here is my 300AW with my D90 and grip, attached 80-200 2.8 AFS, Sigma 24-70 HSM, F100 body with grip, and two SB700's with cases. And there is more room in the front and upper pockets.
Honestly, I didn't care for my AW202. It really wants to be worn the one way. I like my Timbuk2 Snoop than can be worn across my chest or over my shoulder.
I'm not sure if the 300 is different, and it does look like it is very efficient with space (my 202 was, too), but...
I have the exact camera gear you mention and occasionally use the TT Streetwalker Pro. It's a decent solution, but I never got comfortable shooting events with a backpack--perhaps its mostly psychological but I feel like a turtle even though the Streetwalker Pro is fairly svelte. That, plus the weight is carried above your center of gravity, and heat is trapped across your middle back if you're an athletic shooter and tend to sweat. Whenever possible, I ditch the 70-200 for my 85g and switch to my TT Urban Disguise 30 which fits a remarkable amount of gear for its (tiny) footprint and swings nicely over the side of my hip & butt when I'm shooting. That's as unobtrusive a solution as I've yet found. I may consider trying the Urban Disguise 35 v2.0 as it's very deep and would fit the 70-200, 24-70, and a few other accessories for those times when I absolutely need both lenses.
The extra strap makes a big difference, particularly if you ride a bike/motorcycle.
But more importantly - the shoulder straps should not be carrying the weight - the waist strap should be doing the carrying. This a common mistake that I made too. When I learned to adjust the waist strap to carry the load it made another large difference.
Mine contains: 30D body, 5DII body, 17-55, 24-105, TSE24, 100 macro, 2.0EX, closeup lens, extension ring set, three CPL's, six CF cards, angle viewer, both chargers, two spare batteries, remote release, both manuals, 5 in 1 reflector/diffuser, sensor cleaning fluid, sensor swabs, small DOF charts, and a small spare tripod head. My new one is a 302 and has a tripod attachment.
The advantage of a slingshot is that the gear can be accessed without setting the bag down - sure it takes a bit of juggling, but if you're in the field and everything is wet and/or muddy, and there is no handy bench, the SS works where nothing else will.
A bag with a single carrying strap is carried by one shoulder.
For me, because the weight is on both shoulders, high on the back. And it's centered with my body, not sticking out across my back. Makes it easier to walk around in a crowd. The TT SWP will carry my tripod as well, again keeping it center with my body frame.
I'd agree, but it all depends on what I'm looking to do and how much I plan to carry. For example, when I go out anywhere with the 500, it's in a backpack. That lens, camera, tripod+head, 1.4x, etc adds up quickly.
I like a sling when I have a lite kit, camera and no more than 2 lenses and a flash.
Even so, if my use on the lite pack isn't going to require regular access to the bag I look to a small backpack. Easier on the back.
I just ordered the Kata 317 for a day pack/lite kit sling. If anyone is interested I'll feedback after I've had a chance to take it out a time or two. I'm looking for it to carry my 1d4, 70-200 or 100-400, 24-70 and a 600ex. I've been using a micro trekker 200 bp but it doesn't really handle a 1d4 very well.
Fastpack 350 from Lowepro, almost same as sligshot 300 but with 2 straps and easy side acsess...since I switch over this one, I do not have backpain anymore ;-)
I carry my gripped D700 with 70-200vr1, Tamron 28-75 2.8, and have room for a few more lenses (primes) if I want and put my flash in top zipper compartment.
