for several years on river trips. I don't personally think it's worth cutting down. For one thing, the fabric is heavy, and it'd would take more work than its worth to create a new roll top if you decided to cut the volume of the bag down.
furthermore, I think you can save weight, and improve the design and performance of the system for less effort than trying to fix a bulky.heavy/flawed system.
Here's what I did:
I started with a 10L Sea to Summit ultrasil drybag. (for gram weenies, take a lake at the granite gear cuben drysacks)
then, use http://www.usplastic.com/catalog/item.aspx?itemid=23873&catid=669&clickid=searchresults to hook up tubing.
You can go base camp or ULA Amiga style by connecting a hiker cartridge inside the bag.
Or, you can get a lighter faster filter by using a sawyer a few feet below the bag.
I like this system because I can use a standard prefilter inside the bag to prolong filter life.
My filter, bag, element, hoses etc weighs < 8oz total and packs in a sandwich ziploc.