They had sweet potato ('uala), not potato ('uala kahiki). Also, almost all Polynesians had them, so Hawaii couldn't have had direct contact with the Americas.
Anyways, I don't see how Polynesians contacting the Pacific coast has any pertinence with America's incapability to cross the Atlantic.
To clarify, I believe that the current scholarly consensus is that the sweet potato found it ways to the Cook Islands around 700 AD from the Americas. None of this implies, however, and I have not seen any evidence to suggest otherwise, that the Americans were themselves capable of making such long distance voyages. It is possible but nothing in the historical record either really supports it or requires it because the Polynesians were just as capable of crossing the Pacific.
Delaying the discovery of the Americas by Europeans is difficult and this also illustrates why an American-led contact is so implausible. With the closing of the route to the east, the European powers were desperately looking for a way to China and India that circumvented the increasingly Turkish dominated Middle East. In short there was a strong motive for the Europeans to go adventuring and sooner or later, at least as soon as it is realized how far Africa extends south, somebody is going to think about trying the route west. At any rate, somebody sooner or later is going to start wondering why all those Basque/Irish/insert other group from western or northern Europe fishermen keep telling stories about land to the west. Or a Portuguese ship gets lost on its way down the African coast and finds Brazil. Furthermore, by this point, the Europeans have the capability to cross the Atlantic with comparative ease once they know the way.
In contrast, it is really difficult to see why American sailors would stray so far east or west. By the time Columbus arrived, there were two societies with state level organization in the Americas - the Incas and the Mesoamericans - and both were rudimentary compared to Europe. Furthermore, neither (so far as I am aware) had a land of fabled reaches which could only be accessed through a number of intermediaries (thus driving up costs) but which can seemingly be accessed by sea if you sail in one direction. Plus there is no Mediterranean in the Americas - the Gulf of Mexico is closest but that has significant shortcomings - to get a long-distance maritime tradition going and learn how to build ocean going ships. I don't think it is out of the question that Native American ships could have sailed all the way to Europe and they may well have done. The problem is that on a macro-historical level it doesn't matter because they didn't so in such large numbers to make the Europeans truly aware of the Americas nor does it seem that they did it in an organized fashion for trade.
teg