Ive had mine peg at full boost once or twice as well. I think it was due to the boost port that its taking the readings from. or maybe where it transfers the mech pressure to electric at the little box.
As far as the noise...i forgot about that... they make that screeching loud beep at first but you just have to turn it off, by holding the button down at startup, the first time (and if the battery gets disconnected)
As far as accuracy...Im not driving a race car. If i was, yeah id probably go AutoMeter or the like.... But when all your going off of are idiot lights, these things are great... I especially like the spacing of numbers, unlike autometers having something like 15 degrees+ between numbers. Also depends where you are taking readings from....