The DB 605A and AM will do the same power on that manifold pressure and choosen RPM The MW-50 system, 'attached' to the 605AM engine will be used only for manifold pressures above 1.42 ata, when used fuel is B4.
The DB 605AS, the version with larger supercharger, shoud be making more power at higher altitudes (talk 6-7 km and above) and less at lower (4-5 km and lower), due to bigger S/C sucking out more engine power.
edit: Hmm, the lates comment is in practice only partialy true - the data sheets show 1075 PS at S/L for either A, AM or AS, on 1.15 ata. Though, the AS have had lower take off power on 2800 rpm and 1.42 ata setting, 40 PS less.