Depends on the boost level to a degree. When you start to stretch a turbo to it's limit, the engine consumes more air than the turbo can give.

A stage 1 Cossie will still produce decent top end power but on a stage 3, it will start tailing off pretty quickly as you get a sudden surge of power in the midrange then the power drops off as the turbo struggles.

The other thing to think about is the Cossie engine will consume more air than the Zetec engine as it is a better design to start with. The T3 would be better on the Zetec than the YB.