Is this the 'Prince Madoc and the Mandans' theory?
It is interesting and there are some fragments of possible evidence (less than for Icelanders finding Vinland via Greenland, but more than for some random Irish saint who got swept over the Atlantic in his corracle) but surely TBG has got it right. America was 'discovered' by Asiatic hunter-gatherers who crossed the land bridge from Siberia to North America in a number of waves between 14 and 17,000 years ago and over a few more millenia moved south to populate two continents. The Russians got there first!