A little historical context is important. RFK was considered a neutral hitters park back in it's first run, in the 1960's. RFK has not grown at all in the interim. Rather, other parks have shrunk to the point where it is now considered a pitchers park.
To expand on NeedaNewNick's point, if you can buy offense cheaply, you can invest the savings in better pitchers. This is one of the objectives of A's management, as described in "Moneyball".
If it was up to me, I would leave RFK exactly as it is and build the new park with the same dimensions.
This is not only from a "purist view" from a time when ballparks were NOT designed to allow every stiff who could hit a fly ball 340 feet to become the next Hank Aaron, I think its good business sense.
Most teams have now built band boxes and stocked their rosters with guys who are only power hitters in their own and other tiny parks. We've seen the advantage RFK brings (and its not like its a freakishly large park in historical context, just an average '60's-'70's park). Doncha love seeing those fake "power guys" on opposing teams not even make it to the warning track?
A Nats roster stocked with decent pitching and pwer singles-doubles hitters (which used to be the norm in parks like the old Busch Stadium, etc) can put up a great home record every season (or until others move their fences out). About all Nats need is their very own Jack Clark or Frank Howard: a guy who hits monster home runs that would clear the fences in any stadium. I suspect this is why Bowden covets Willy Mo Pena with his 500 foot power stroke; there are other guys out there too.
If I was Bowden tho, I'd acquire more speed (and maybe a different manager who actually likes to send runners). Think the Whitey Ball Cards teams of the '80's. To me that's a lot more exciting than waiting around to see some lumbering, defensively challenged stiff hit a 340 foot home run.
And despite MLB's current view (which, in addition to the encouragement of tiny parks resulted in the juiced balls of the Maguire-Bonds record years and the tacit encouragement of steroid use) I think fans are just as happy to see exciting, winning ball where home runs are an event to be cheered, not something that happens 6 to 8 times per game.