I have a feeling it's not as big of a restriction as the intake manifold is in general.
That was certainly the situation with an engine I'm familiar with, the Fiero's 2.8 V6. It used to be a popular mod to bore out the TB... until it was clearly demonstrated that the major restriction was actually in the "neck" of the intake.
The first image below shows where the intake was cut open, and the second image shows a stock TB gasket placed over the end of the cut intake. In this particular example, it's quite obvious that opening up the TB was pointless unless this major restriction was also removed from the intake.