Is it intercooled or not? Right in the intake horn would be a pretty good place. Check to make sure you gauge line isn't kinked or pinched, make sure you dont have a boost leak at the boots or intercooler? If everything is good than check for exhaust leaks, an exhaust leak that would cause that much loss would deff be noticeable though.
2001 2500 4x4
, DDP 150, TST Comp/Smarty, S&B Intake, PSC brace, Hamilton 110 springs, Fluidamper, ARP studs, 4" straight pipe, AIRDOG 150, BD black FP, Suncoast billet trans, 3-disc converter, 3rd Gen Brakes, Thuren 3rd Gen track bar. RIP grandpa "Red" 10-11-08