Steve,
Just think of an engine as an air handling unit, the bigger the engine the more air it can process - 2litres of air is less than 5.7 litres of air .

A T3/4 can barely give a 2 litre engine enough air for 400bhp. Putting a T4 on an engine that goes up in air flow capabilities the same as the turbo does will result in the same boost drop off, as the bigger engine will be able to consume more air.

The thing that most influences the power with a turbo is the VE of the engine. The Cosworth has a pretty good VE when compared to others .