Massive can of worms this one.

Many people are adament that the T3 is a pretty robust turbo and can manage without one even in the higher boost figures.

There is the argument that ford never fitted one so they dont need them, with the rebuttal being most people are running much more than standard boost.

I personally found the only difference running a DV is the lag between gear changes. Yes the DV releases pressure and allows the turbo to free wheel which would reduce lag, however the turbo then needs to repressurise the intake which has had all the air "dumped" out of it.