Swapping a jdm turbo engine into my s5 . I was told I could not spool up the turbo. Unless I got the stock turbo exhaust. Or I got a standalone to use with my aftermarket exhaust.
So does this sound right or is the guy just trying to get me to buy a standalone ecu so I can spend more money with him?

The engine is all stock even the ecu.

 

with an aftermarket exhaust, your car will make more boost and the fuel cut will come on.

You need to get a fuel cut defender