In my case it was, as the FP green had a larger inlet than the stock up-pipe. So I ported it to utilize the bigger opening. So the stock p20 housing was smaller already because of this.

The inlet to the turbine housing is between 0.100" and 0.125" smaller than the outlet of the stock headers (..on the radius, about a 1/4" on the diameter).

I wouldnt bother porting if the pipes matched.

Take a look at this way:
In my case u have to look at it from the begining... when i ported my headers, and changed to a 44mm cross-pipe - the opening of the headers where smaller than the block. So porting here was essential. This point here created more flow to the turbo.

Getting rid of the 32mm restriction in the middle to a 44mm x-pipe:
area = (pi)(radius)² = (pi)(diameter)²/4 = 0.78539 (diameter)²
a = (0.78539)(44mm)²
a = (0.78539)(1936mm)
= 1520.5 millimeters squared - for larger opening in headers

compared to 32mm crosspipe restictoin:
a = (0.78539)(32mm)²
a = (0.78539)(1024mm)
= 804.24 millimeters squared for crosspipe

(804.2) / (1520.5) = 52.9% reduction in airflow (very roughly!)

take away 15% losses and shit + or - on top (thermo/turbulence/cast iron resitance etc etc)

u can see how much damage it does even if this quick calc is just a rough guestimate based on the pipe mm opening. And yes its about port matching as stacey said.