The easiest test to find out if the dump/exhaust is being a restriction, is quite simply to drop it.
But can anyone find a car make 400RWKW with a 3" dump? Most I know of from experience run 3.5" - 4" dumps, and then a 3.5" system.
While you're say going to a smaller pipe increases speed, decreases pressure. You also have to know for a fact, that whilst you go really small, the speed of the gas might be super quick, and low pressure in that section, but the section before has been restricted down basically, and it now has a huge pressure build up.
Run too small of a dump, the pressure in the turbo / exhaust manifold starts to build up, and counteract the pressure going INTO the motor.
It's not like I said go and put a 6" dump as from experience, everyone knows that it will become detrimental to the exhaust flow.
Whilst the hotter gas is less dense, it also takes up more area. And that's the big problem, as it's exitting the turbo, exhaust gas can be as hot as 850degrees, by the time it reaches that tail pipe, a mere 200 - 400 degrees.
Half the temperature, half the density... So you need half the area for the cooler gas...
I agree, there is an art to building exhausts, bigger is not always better, but experience tells me, for 400KW, a 3" dump is too small.