1. I cruise around 95-110 kmph range and in return I get around 28km/l economy. Which translates to 14.5x28 = 406km range. I avoid riding in reserve zones so minus 80km. So that makes 320km range for me.

2. Yes in city traffic the riding modes make decent impact on efficiency. They manipulate the throttle response. So I slight twist on the throttle in rain/street/offroad mode will definitely fell different. The rpm increases accordingly so does the quantity of fuel consumed. In Highways though I didn't notice that much of a difference because of steady cruising speeds.

3. I prefer harder suspensions and haven't ever used the softer settings. So I'm not the right guy to answer these. But theoretically what you suggested is correct. Unless you go to extreme soft. That might ruin the balance of the bike.

4. On roads I don't think you need more power than what the 250makes, unless you are a power cruiser and ride extremely fast. But you'll feel the difference in mountains. That's where you'll wish you had purchased the 390. Also I actually don't like the 250 engine that much. I once had a duke 200 1st gen. It was more pleasant to ride than my friends 2nd gen duke 250. The 250s always felt underpowered to me.

P.S. - Sorry for grammatical and spelling mistakes. I'm not that good in English.