Who said anything about "buying a prebuilt kit"???
The point would be making your own rear mounted kit would be extremely CHEAP!!!
I made a design for twin turbos that would not cost much over 1k.
2 small turbos spooled up by true dual pipes. Low boost equals instant response. 5-6 psi on 2 small turbos is going to be the 440-460cfm range. Almost the same as a T28 at peak boost. Though doing it with much less heat introduced into the air charge.