Thanks for the replies guys!
Skipping the stuff sack for my sleeping bag might be an option, but I think I would have to go for a dual compartment frame bag to have a "clean" and "dirty" compartment. I have done some testing with my sleeping bag(400g down summer bag) and the supplied stuff sack, and I can actually get a pretty narrow profile of it with a bit of persuasion. I want to keep the tent as low as possible for handling, even if it is a light one(
http://www.helsport.no/ringstind-superlight-1-2 dual skin, under 1kg). The tent packed is shorter than the down tube, so I think I'll be able to fit it in there as well.
About water, I'm thinking a camelback or similar instead of bottles. Takes up a lot less volume for the same amount of water, and a lot of frame bags comes with a hole for the tube as well.
I'm still waiting for my new bike that's lost in shipping somewhere it seems. When I get it I'll get a better idea if I can fit what I need without a front bag.