Yeah, we have a similar setup for haberdashery - white Ikea Billys, a few extra shelves, and a lot of 9l RUBs. They're a couple of inches deeper than the shelf, but that's not really a problem. Larger projects and long-term storage go in 35l's or 64l's. The 84l's tend to be too big for us - we've got one, and I hate lugging it around.
That's a hell of a collection. Is there any risk of them degrading over time, as they're organic?
I discovered recently that IKEA actually do a deeper Billy that you can order online. I've seen them as deep as the Kalax. I used to work there and never knew. Might be helpful for the overhang