4 mins? I don't know how long it took me but I left it for a minute to go do something. I think it was less then that but i didn't look.
Considering how set you are on supercharging your BMW the easiest option to me is starting to be one of two options: S50/52 swap or M3! I kinda licked out with my 328i as performance upgrades are readily available. Same with the 325. I fully understand your quest was a supercharged 318 but availability of parts is pretty small.