1421 with a twist?
Hmm, sounds a bit like a modified Gavin Menzies theory to me, though on his website there was an idea there was some 'evidence' that the Chinese visited Ireland during the 1420s (I never got to the bottom of that one, I guess it must have been a ship or ships that got a bit lost either on the way to America or on the way back when the rest of the fleet went via Greenland-if you accept his claims).
When we say Treasure ships, we do mean the nine masted monsters that could carry 2,000 tons of cargo, 500 feet long, 40 feet high rudders (eg almost as long as the Nina), etc? I seriously doubt the shipwrights of Europe would be able to even attempt to build anything on a similar scale-I think the largest European ships of that time were 150-200 feet long (please correct if I'm wrong on that last point).
My own feeling is it would have seriously have shocked whoever caught it, I wonder if anyone would have had a port or dock big enough for one? They might have had a go at trying to copy some of the ideas (maybe multiple hull compartments, etc) if they realised what they did. However the biggest effect is it might have scared the Europeans, they might not have tried to sail eastward on the evidence of such giant ships.
The more interesting 'what if' is what if Gavin Menzies is correct (basically the Chinese visit America, Australia, etc in the early 1420s) and then carry on, establishing colonies (though small and not as part of some attempt at colonialisation) and perhaps help the various native people to develop, even giving them firearms? If and when Columbus et al turn they find some lot more advanced cultures in the new world and far east and the European Conquest fails or just never gets started...