15psi on a upgraded turbo? sounds liek lack of fuel to me. ive run 18psi with my 57 trim ct26 and stock mkiii i/c. ive spiked to 25psi in 2nd gear once. forgot to hook the wastegate hose back to the boost controller after doin something to the car. i knew somethin was up when i smoked the drag radials from a 20mph roll in 2nd and didnt floor it. looked at the gauge and said oops and let off. still fine. if you dont tune it right even a mhg and arp's wont save you