the IS38 has a slimmer turbine shaft, with bearings with too much space, so when overspeeded (when the tuners pushes the boost up too much), they tend to break..
Unfortunately pretty common when pushing these turbo's..
Get yourself a Littco replacement(the L380x has uprated bearings and no more weak teflon seal) and then you can enjoy the full 400hp, or go bigger and get the L460 (for 450hp) or the L480 (for 500hp)