The taper promotes a less dramatic pressure drop in an area which has been identifid as being problematic by testing on this very car.
Ever wonder why your making custom parts that no one else is making?
On a turbocharged car everything is about pressure drop. You WANT the pressure to drop across the turbine.
What he has done is probably better than a smooth taper.
And there is absolutely nothing smooth about what is exiting the turbine/wastegate.
It would be better still upping to 4" for the downpipe and then downsizing. As Rod as already suggested even around 6-700bhp, 3" isnt really much of a problem. Although again going upsize certainly wont do any harm.
But if you had only 4-500bhp or so, it would be a waste of time.
Back pressure in the exhaust really doesnt sap power as much as you think, unless it was silly.