A better sensor to use might be the one that drives the boost gauge. It
has a resistance that varies from as the intake pressure goes from -0.5
bar to approaching +1 bar. Current is supplied to the sensor through the
boost guage and a proportional voltage can be externally measured on the
wire that connects the two. This wire could also be connected to an
external device, as long as it is hi-resistance so as to not load things
down.
A good candidate for this would be the LM3914N that is used in the DIY
air/fuel mixture meter that drives LED's. In this case, it would be
used to drive LED's or ? as it goes from vacuum to boost. These "trip
points" could be diode or'd together also.
One example would be to "or" all the boost points and use that to turn off
the A/C compressor on the assumption of when you have boost you want max
power.
Another, would be to pick a high boost point and drive an "overboost
alarm", light or audio alert.
http://www.ncf.carleton.ca/~bm959/afr.htm shows show of what this might
take. A resistive divider would have to be added to the 3914 input.