Future contracts depend more on the position of a country than on its payment history. Consider Ecuador. It has better ratings and access to credit after it pursued aggressive debt haircuts.
The difference is that the only reason Greece is able to loan money at all is because they are lending from the ECB/IMF at below market rates, an option which they will no longer have when they leave the EU and default on IMF loans.
A non-difference is that most of those loans went to service old loans anyway. And the main reason Greece can't get loans on the markets is because its current debt is obviously unsustainable and its position as a deficit country in a badly designed monetary union is hopeless.
You hit the nail on the head with the position as a deficit country: Greece will have to solve that whether it stays in the EU or not. The question is which way to achieve that is harsher: the political force of the EU, or the market force Greece will have to endure when it goes back to its own currency. I think the latter will be harsher, since nobody has any incentive to keep Greece afloat other than if it generates a personal gain, whereas if Greece stays in the EU then countries like Germany have a greater interest in keeping Greece afloat to prevent destabilization of the EU. Therefore they will funnel money into Greece at rates that have negative return on investment if you look purely at Greece, but which will have positive return on investment if you factor in that it keeps the EU stable. The market however, has no such incentive.