As others have said, regarding the capacity, and just picking figures out of the air, if you had say, 2 x 50 litre cases, they wouldn’t necessarily fit into a 100 litre roofbox because of the tapers at either end, especially the front. Most often, they work well for loose stuff that you can squash in around one or two big items. Just make sure the stuff you want to carry in the box isn’t the stuff you’ll need access to while on the move, and IMO, the locks would be easy to break for anyone determined enough, so maybe have a think about what you’re putting in there.
Excellent load increasing kit all in all – just require a bit of planning to make full use of them.
And +1 for roofbox.co.uk – they do provide a good service and quick delivery.