OE: The OE part has a 50mm (1.94 inches) butterfly.
The open end toward the turbo is slightly larger at 2.10",
but this isn't important.
Aftermarket: The stock TB is much larger than the turbo outlet, it is also the one that the EEC is calibrated for. Changing to a larger bore will cause lean WOT and will not increase max power.
Modifications: Probably none is required; however blending the surfaces never hurts. Putting a full radius on the leading edge of the butterfly and a knife edge on the trailing edge may help. Grinding the screw head lower and reducing the throttle shaft thickness also may help. When grinding on the butterfly, DO NOT grind the edge itself otherwise when it is closed it will allow too much air for idle.