boost/overboost sensors?
 

guys do these sensors determine when the boost comes on?

In a 930, no. It's a saftey switch/relay setup to cut the fuel pumps should you exceed a certain boost level (not sure what that is). On a 930 model, the pressure switch is located on the large BOV housing on the driver's side.

Things (efficiencies) like your turbo, headers, I/C, etc. determine when you actually begin spooling (boosting).

How quickly you are attmpting to accelerate also plays a part. If I stay easy on the gas, I can keep my turbo (for the most part) from spooling. If I really get on it, that lovely whistle begins to sound!

Keep in mind that with turbocharging, the first line of over-boost saftey comes from your waste gate. The larger (heavier) the spring, the more boost that's allowed to flow/accumulate in the charge system. The lighter the spring, the quicker the boost begins to dump/exit.

Stock 930 overboost switch cuts the pumps at 1.1-1.2 bar. If you replace the stock spring with a 1.0 bar spring you may bump the overboost switch. Just one more reason not to do that.