I'm not sure that we do need such a term. Ability is a continuous function, and there's nothing magic that happens once you reach 10 times the average[1] that justifies such fixation. I never hear anyone talk about recruiting 5x engineers, even though it would be more viable as a business strategy. Besides, we can't even measure engineering ability to the precision at which specifying "10x" even makes much sense. When would "10x" actually be a more useful descriptor than "very good"? Also, where are all the "10x artists" or "10x mechanics"?

[1] Although I read here just a few days ago that a 10x engineer is only ten times as good as a bad engineer, not 10 times the average. So maybe it hasn't always meant the same thing?