Ok now I’ll admit I may have taken my "I" a tad too far with the M rear duals and body panels and such but when you are talking the power train it’s a whole new beast...
when I was converting my "I" to dual exhaust and going in and out of the "I" "Si" and "M" models there’s a lot going on in their than just the engine, first you’ll have to buy other suspension bits and fuel hoses and then you’ll have to get the exhaust correct and that has a whole set of issues and if you’re not doing the work yourself and you pay someone 30 bucks an hour you’ll quickly get up and over the cost of an M.
if you look around you can grab an M for 5 to 7 grand more than your car now, some were way less some more just look around...also you have to replace the ECU and gauge cluster tied to the car as well as key and all that jazz, you’ll also have to have the title and such to the engine you have in case you ever lose the key as BMW needs proof of ownership.
like I said its cost effective to just buy an M, since this is my wife’s car that I get to tool with it. Its an auto so I’m just having fun, I know I’ll never buy the M and I’m fine with that but I can play with the looks of it for now. not sure if you have the Si or I but if you want to wait I’m having a special tune done that will get your I to be better than that of a stock SI for HP numbers and from what I hear the feeling from and SI and an M are very close to one another.