Help me understand HP potential of S vs TT

TopgunZ

I know im probably going to get my balls racked here but I am just confused.

I drive a 350Z with the Powerlab kit with a single GT3584R. It is capable of putting down 550whp. And its a single smaller turbo. And there is room for more.

I am looking at selling my car and purchasing a 370 in the near future and am excited yet I want to make sure I get the right kit on there the first time.

I inquired into the GTM stage 2 twin turbo kit. The sales rep shot me back an email stating that at 550whp I would be maxing that kit out, the GT28's with the .84.

So I am just confused as to how you can have a single turbo, that isnt all THAT much smaller than a GT28, making the same power #'s as TWO turbos of a slightly smaller size, on larger engine. Has it got all to do with the fact that they really do just drop off in efficiency at such a lower psi?

I would like for there to be room in my system for growth and I also wouldnt want to be pushing my turbo's super hard all the time.