Thanks for starting the thread Richard! its going to make interesting reading I am sure.
I was reading about a kit from VF this morning thats about £4k for a supercharger for Z3 2.8/3.0 that adds approx 86bhp and can be fitted in 10-14 hours, so for me that's about 80 hours with my limited engine knowledge (Escort Mk2's mainly, from a long time ago).
It also needs an ECU remap.
I like the idea that you can bolt something on to get more power, rather than major mods.
As I have changed to large wheels acceleration has deteriorated slightly, so it would be good to mitigate that with some extra power.