It´s because I am using a T61(about a GT35) and even this one is limited to around 18~20psi with a maxed out MAP ECU and the AFRs in the low 11s.-At least in my case.
Therefore I thought/think that an even bigger turbo like the GT40 is limited even more at lower PSIs and so it´s not "optimal"?!
But I could send you my map if you want to try it out just LMK?!