Any question answered...

I've never used anything other than Shimano. They stop fine if you keep your rims clean. I'm assuming the main benefit of the more expensive pads is that they leave less residue?

less residue, less wear and tear on the rims, longer lifespan, better performance in both wet/dry, basically the single best improvement you can make on your bike, people spend hundreds of pounds on upgrade but never thought to change the simplest thing; the brake pads.

Get some cheap Kool Stop Salmon* you'll notice it, in fact it should be the first thing you upgrade for a new bike, don't buy a new brakeset, get a new brakepads.

I'm just wondering whether the Swissstop Green is worth going for despite being twice the price of the Kool Stop.

*having says that, the Salmon itself wear out slightly faster (although I have not noticed that, to be precise can't really tell the difference), but still offer better braking.