I did a search to see what the 511 pack you’re referring to looks like. I got a few results..I dont know modern kit but my friend uses his 511 ruck. I keep expecting it to bust with as much garbage as he stuff inside but its stood fast for 2 years now. He seems to like it though I suspect he tolerates the size rather then enjoys it. It does have loads of places to stash kit and he spends a lot of time searching for things.
I have the full sized Bergan. The Snugpack products are copies of the British Military gear. The packs fit well over your kit, hold fuck all for gear, and are reasonably priced. If you get the yolk you can take the rocket packs and make a day pack out of them.