IIRC Settings 1 & 2 in the manual was reserved for riding on smooth roads, where i believe it would only beneficial for comfort. But I ride with some aggression, so I actually prefer the scooter to track and steer positively. I have ridden about 2000kms each on settings 3,4 & 5. 1 & 2 is to me rubbish. 3 would be the overall average. 4 is a touch into better handling and 5 is you want max response for flicking left to right.
The higher you go of course the ride gets harsher but the steering and handling somewhat increases. The DT is heavy, over 200kgs, the higher settings are required to control that weight. Again, it would be nice to actually have rebound and compression adjustments to control that spring rate for a really refined ride.
OE spring rates always are determined by the manufacturer on what they think is the average that their customers will use the bike for. It is not what is best for the individual.