You can surely put more on an S40 for touring than you can on a V20, but my V20 is set up with a nylon material 14-16L expandable seat bag from a sport bike. I use the Performance Adjustable head rest, as well as some TT bars from my old TT bike, and this allows me to clip the bag on or off in a few seconds, and it is not only secure, but quite water resistant. Also, it is aero, but that is another story. Here are a few variations I have had over the last year or so. The bottom two are made of fiberglass, the small one is about 8-10 liters, and the big one is about 20 or more.
If you wanted even more storage space you could put a top tube bag on the boom for another 5-6L, and you could also attach top tube bags to the seat, giving you about another 4L of storage space per side. I think all of those options are going to be the least penalty aerodynamically, but you have to think of the total volume of each of the things you plan to carry.
For hydration, I wear a 6L jogging backpack backwards on my chest, and put a 3L hydration bag inside with a bite down drink tube. It also gives a little bit more space for wallet, iphone... etc. It is a little different than this one, a little lighter too, but I really like it because it also serves to cool me down in Summer by adding ice to it. You can't see the drink tube in this pic, but it is there.
