A few setting in the PCM and the actual MAP sensor.
BUT there is an issue that noone has solved yet (AFAIK)
The L36 measures barometric pressure whenever you're at WOT. So if you have boost at WOT (and the PCM knows about it due to the 2bar MAP) your baro reading gets all messed up and your idle will suffer after going WOT (to the point of stalling in some cases).

Unless you're going to do a speed density setup just stick wtiht the 1 bar until someone figures this one out.
I found this. It does seem plausible now. May explain my super rich decel, too. Think I still have my original map.