set up is 2000 Silverado

Stock bore 5.3, gen IV rods, arp 2000 rod bolts, forged pistons rings gapped to .028
706 heads, arp head studs, ls9 gaskets
texas-speed 224/228 .600/.600 114LSA
TBSS intake w/ 92mm TB
huron speed turbo kit w/ 78/75 billet vsracing turbo .96ar 3" downpipe
4l80e trans with a 2800-3200 stall

on the dyno at 6-7psi it made a whopping 388/399 with 18* degrees of timing and was not really making much more as boost increased. as we got to 15psi with 14* it lifted both heads and a nice little fireball from the engine came out along with all the coolant. made 485/515

what could possibly be creating so much pressure in the cylinders to cause this? im sure the turbo is big enough to not cause a restriction. its wayyy down on power in my opinion

all motor with the factory intake and TB and a smaller cam it made 315/298 moving heavy 22" wheels

any help is greatly appreciated!