A standard map sensor only reads to 200kpa absolute- so even if you run 3 bar boost for instance (400kPa absolute) the map sensor will only read it as 200kpa, and give fueling/ignition to suit- which isn't good!
We sell a 250kPa sensor, allowing you to run 1.5bar boost (21.75psi) which on a 1341cc G40 will net you over 200bhp on a K03
The switch you talk about replaces the full throttle switch with a boost switch set to 5psi, this allows the enrichment section of the map to be achived without the throttle fully open, which you need with a turbo.