I also have the Fastpack 350, but to be honest I don't think all of the things you mentioned really fit well in the bag. Unless you don't mind flat sandwiches The bottom compartment is big enough for your camera+lenses+flash that you want to take, but no more than that. The top compartment is too small IMO to fit 3-4L water, food, clothes, tripod, first aid kit, flashlight etc. It might all fit, but it's going to be very tight I think.

The F-Stop bags seem pretty cool and bring me to an idea: What about a dedicated hiking backpack with a separate ICU (from F-Stop for example).