Yeah now I remember that being the case with the instructions. I think I got all of my probe install info from this site IIRC. Good luck finding the right tap, I know I had to go to three different places to find a NPT tap. Some places just don't carry them. REmember must be NPT.
As far as my boost goes, I can get it up to 30-31lbs. I have an Edge box and a wastegate boost tube. Normally on a hard accel. it will be about 25-28lbs. with an SES light at 24. 15 seems a little low, are you stock? I can't remember. Is that at WOT?