An HX40 is not the best choice for this application due to considerably heavier rotating components. I think it would be pretty sluggish spool wise? Bill Heath had me look at a T04 "P trim" turbo someone else made for him and he said he made 440 horsepower on it but it was sluggish as a turd. It was supposed to be a Garrett turbo and it was definitely not, it was a Borg Warner version of the Garrett T04 P trim. I spend a lot of time weighing rotating parts because this is a huge factor when looking at spool rate. The "P Trim" turbine is not a great match for this without going to a considerably smaller A/R than what was being used on Bill's sample turbo. I am a firm believer that if you have an electronically controlled engine package underhood (GM5-GM8) you need to see Bill about his tuning package or you will not see what you want. Careful parts selection, time , and patience are what is needed to get a combination that works properly. Nowadays everyone is a turbo expert, everyone except the turbo specialist himself.