You will need a speed 6 ecu and mounts at a minimum. I know of one guy in Europe that swapped into a 6 wagon and did not add AWD but I think he was starting with a wrecked speed 6
Trying to answer this has made me realize I am exhausted and its time to stop doing chores and call it a night.
I have no idea it doesn't seem like it should work but if it works for the brakes? Idk
Your idea is how I would proceed also. I agree with Derrick and penguin that the alignment will probably unlock a lot also.
Don't underestimate the impact of different tires
The goal is to go faster so work from the new setup to make it feel like you want dont work back towards the old setup. If you had the front at a good height then raise the rear. You can be too low and outside of the optimal range for the suspension
Wait if you were as fast with the new springs as you were on the built motor doesn't that mean the setup is faster even though you don't like the way it feels?
I have the street bushings for what its worth
Honestly for the amount of labor involved in making the switch I feel like its a buy once cry once scenario. Unless you are seeing tons of great reviews on the cheapest kit I would spend the extra money for a known quality.
Also lifetime warranty...