I have the Osprey Porter 65 and it's wheeled cousin, the Sojourn 25". I've used both of the for trips of up to 3 weeks. I had the Porter first - when I decided I wanted a larger wheeled bag than my Eagle Creek Tarmac 22", I got the Sojourn. I also use the packing cubes and it does make things easy to pack and find. I don't think you can really overpack the Porter - It isn't a true soft side - the bottom of pretty rigid and the two long sides are a little stiff.
To the OP - if you want to add a little to the Porter, check out the Osprey Daylite daypack. It clips onto the top of the Porter so you don't have to carry it around until you want to. It would make running through the airport with that Starbucks in your hand still pretty easy :-)