I was messing about with my mates sti that's on tein coils and 360 bhp ! He was shocked how fast I went round the bends with my wagon with less power , not on coilovers and not as good tyres
I've had both (2 saloons and 2 wagons. ) I added rear strut braces on the wagons and whiteline rear arbs and it holds the road like glue. !!
My saloon handled well too
My mate swears the wagon handles better as it's more rigid over the rear wheels
Didn't get anything done but my dad jet washed it for me ! It failed mot on brakes so it's going back for a retest Friday after my new ones arrive tomorrow
Then it's up for sale
Map it in by all means mate , your mapper will know what they are doing
Years ago I had one on my vectra , allowed me to select low boost when it was out my hands
Yes is the easy answer
If you want adjustable boost then look into a bee r limiter or apexi unit but you need to know what your doing and be able to read air fuel mixture readings