The spool I would vote the biggest obstacle. You keep upping pressure, and you will keep getting more air into the chamber. You can increase flow as much as you want, but I almost gauruntee it won't change but a few PSI of difference. Take turbo diesel. They have no problem with 45psi or higher. Though witha compound turbo system you might actually have to use an intercooler between stages. That build could get so ridiculous looking..