The GS brigade are what the roll tops were designed for, after canoeists, yatchsfolk, polar explorers etc. etc. and I bought mine when I had an 1150.
Also, having used the BM inner waterproof bags in the Sports Cases I am well underwhelmed. They are stiff and 'orrid and take up more room than the stuff you are packing, good build quality, just a tad over-engineered.
I use some Mane Event inners which aren't waterproof but are light and sturdy and easier to carry into a hotel than the cases. Wunderlich do some lovely stuff sacks which are totally waterprrof and easier to compress than bin liners, although I always have a couple of those on board somewhere when on tour.