E46 M3 ECU options

Hi all,
Sorry i did some searching and couldnt find a clear answer for this (forgive me if i miss it tho).

Basically im wondering how far you must go in order to alter various parts of the ECU. Are they flashable? Im from the EVO world and Megasquirt world (on an MR2) and ive done a lot of self tuning in the past on my cars and am fairly experienced with it.

Im wondering if the stock ecu is flashable in any way, and if its possible to get the software so i can get onto it. I dont expect anything as advanced as an EVO ecu (you can wire in widebands, flex fuel sensors etc and completely modify basically any parameter of any setting) but just want to understand as much as possible. And yes, i have tuned a honda with ITBs using alpha N and then upgraded to ITB mode on a megasquirt, actually worked very well.

I simply would like to get a bit more out of the engine with an exhaust setup and a better airbox and possibly cams (my reading tells me this is the best direction for a basic NA setup for some power gain).

Any help is much appreciated

sorry mods,
may as well close this thread, did some more reading and reflashing seems pointless. It makes no sense to me how all these aftermarket companies and somehow make new tunes and sell them for 100s, but someone at home cannot to it.

standalone aftermarket seems the only way to go

BMW made their engine computers hard to crack, they don't want people tuning them. Japanese cars are more open to tuning.

but i dont get how all these companies can offer flash tunes. Those tunes HAD to be made somehow? They had to be able to view the entire map and tune it in all respects.