Thanks for everybody's thoughts.

Putting 3kg of weight on the rack isn't an issue in terms of handling, the bike handles just fine with a single pannier on it so I reckon I'll be fine with the weight balanced. I don't like backpacks because they have always made my back particularly sweaty and it's hard to see how anything can sit on my back without making it sweat. I'd also want to check access to my jersey pockets if I had anything with much size on my back. On a long ride (i.e. the times I'd want the extra water) I wouldn't necessarily be pushing hard in terms of bursts of speed but the sustained effort usually sees me quite sweaty. Another consideration is whether a waterproof can fit over me plus the backpack because the last thing I want is to be faffing with taking the pack off, putting a waterproof on, then putting the pack over the waterproof, if it starts raining.

The hose near the spokes is an aspect I hadn't considered, if it did detach from me for any reason that could be a problem.