I guess I got confused since it seems to be used for emphasis, as though tenfold really meant more than 10x. Anyway, glad to get that straight.
That's cute, haha!
Yes. It's quite commonly used in British English afaik.
You can say "This brought a twofold increase in clientele", "We saw a threefold weight increase in affected mammals". I've never seen it used passed "fourfold" though.