Don't know as much about this as some others on here but here's my considered 2 pence worth!
1 - I have an SPS switch but don't fiddle with settings as I have no means to measure the changes. JKM went to some length to set mine up so I'm not monkeying around with them.
2 - Do I/would I drive hard enough on the road to feel relatively small changes? Not a chance!
It's already bloody fast and I have far too many 'spirited' excursions
3 - I believe that ECU will take care of optimum settings in a tuned state.
4 - Don't want my boost running any higher than at present. My turbo is currently at c60% duty cycle which means it's having a relatively easy life. DSG map will see the car settle at around 320/330bhp (currently 313) as rev range is extended since it continues to make power right up until it cuts. These are good solid figures with hardware that is arguably over the top for stage 2 but it does mean that for sustained use (tracked occasionally & fast road) the engine will not become impeded through heat or fueling issues for example. I also think this approach is valid at all tuning levels.
I'm sure there'll be many opinions on this!