I can't comment on Roof Bags but I've just bought a Roof Box & have been amazed by the amount of stuff you can fit in one. Watertight & locked away solid. Expensive though.
I got a Hapro Traxer 530L. I've learnt that the way to fully maximise the space is to have lots of small things, rather than a few big things in it. I think you'd make it fit the 4 x 80L packs, but you'd probably have to store (some of) the packs empty & flat, then throw the food/sleeps sacks on top.
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