It's my impression that the term MacGuffin is going through a period of increasingly inaccurate usage to mean: the object of the quest or really important object that drives the plot. Originally, I take it, the term was invented to apply to such an object where the nature or identity of the object itself did not matter. You could exchange the Maltese Falcon with the Siamese Cat and change the figure from black to pearl and the source from Templars to a Wuxia order and it wouldn't matter in the least.
If the artifact itself matters a lot, then it seems inaccurate to call it a MacGuffin. Reportedly, there is some disagreement on this score. Hitchcock famously said the audience doesn't care what the thing is, but Lucas and Spielberg seem to think the audience should care. I take the Sampo and the Golden Fleece to be MacGuffins. But things like Sauron's One Ring and Holy Grail matter a great deal to the audience and the elements of the story itself beside simply the action of the plot, which would not make it more accurate to call them something else. They are Plot Devices, but they are also important to setting and characterization in critical ways. The term Meta-plot Device appeals to me, but it may resonate too strongly with the usage of metaplot to be useful. What would you call such artifacts?
For quality time with Wikipedia and TVTropes, check these links:
MacGuffin: Wiki; TVT
Plot Device: Wiki, TVT