If the MAP sensor was truly knackered then from my experience the car goes into limp 'get you home' mode. Doesn't sound like it's that bad from your description. Is it possible it's just the dash gauge which is knackered? I'd need to refer to the BGB to trace where it gets its signal from. Like you say, you don't want to take any risks by boosting and risking lean running until you get to the bottom of the problem just in case the ECU is taking this erroneous boost reading and matching the fuel to it.
It's handy having a spare 205 in the driveway for swapping components!
Not much help I know