"...they will have to run stadium specials to cover the crowds at those games."
Not if the Expos continue playing as they have been doing.
Flex-rush hour for fans leaving the stadium can be instituted starting in the 7th inning!
As for stadium specials, they will be impossible to run on the Green Line unless they build a connection to the Yellow Line bridge or something bypassing L'Enfant Plaza. Besides which L'enfant Plaza has no cross-platform transfers from the Green to the Yellow line.
There are no pocket tracks on the Green Line south of Mount Vernon Sq.
This fact coupled with the mayor's "economic development" plan (tear down the Nation nightclub and build a huge parking lot between the station and the stadium to "anticipate future development"),
coupled with suburbanites reluctance to travel anywhere south of L'Enfant Plaza,
suggest that the new stadium would be an absolute bust for transit, and possibly for the owners as well!
I say keep the team in RFK and renovate it completely, as Jack Kent Cooke proposed to do with his own money.