Jedon

What is the difference between a large turbo that flows a lot of air at lower pressure like a T88 at 15psi verses a smaller turbo capable of higher boost that is smaller? There is a guy on PartsTrader selling a Garrett TR30R from a WRC car that runs at 3 bar, but only has a .7 A/R. I understand that turbo's like the HKS T51R can run at 3 bar as shown in that Scoot video but that is a large turbo. I guess what I am asking is if it is only the total amount of air that matters, does it matter how that air got there? I want 450RWHP, what boost level would I need to run a TR30R at to achieve that, with a brand new streetported engine with all the other stuff like fuel system, CPU etc? Thanx!