Empirically, the case of Japan contradicts your claim that there isn't a whole lot of room to move. To remind you, Japan has long at close to and above a 200% debt-to-GDP ratio without any financing difficulty in sight.

The underlying reason for that is plain and simple: both the Japanese and the US governments are currency issuers. By definition, they are able to spend as much money in their respective currencies as they like. In that sense, they always have an infinite amount of room to move.

The only question can therefore be: How much of this maneuvering room should be used?

The answer to that question does not lie in financial data alone, but must take into account the state of the economy at large and whether additional spending might hit an inflation barrier. In the current situation, spending directed towards job-creation is unlikely to hit such a barrier, while handing out more money to the rich could drive speculation on resource markets (the money will be "invested" somewhere, after all) that can cause price increases and thus inflation.