I don't see any way the WRX spoiler will work. The WRX is considerably wider and the rear window to roof line area isn't even remotely close to the same shape. I would forget this project and add the M-Sport S2000 wing that actually functions and is designed to work on the Fiesta.
I suspect the revodevilcustoms wing would have a similar, less pronouced, aero effect. It has a pretty good placement (more or less in clear air flow aft of the hatch) and a reasonable shape (low angle, nearly flat, upside down wingish appearance...can't tell for sure).
I love the M-Sport Fiesta wing (R5? S2000?, whatever they call it...Team O'Neill can provide the details).
I recall former FiestaST.net user kamakazee did some fantastic modification work on a white ST and used that wing. It is not a cheap route to go, considering the requirement of swapping out the stock ST wing for a stock Fiesta mini-spoilerette (required for through=the-wing mounting purposes). Not to mention the price of the thing itself.
I do have a concern with either of these wings applied to our car FWD car without any thought about the nose.
A truly functional wing will apply pressure to the rear of the hatch. More speed, more pressure. That load at the tail should be offset by meaningful downforce on the nose. I'm not thinking of the cosmetic commercial "splitters" available to us (Triple R, mountune, etc.), but a splitter with some tray behind it. It would almost certainly need to be a custom deal. Together, they can help tame a car that has substantially added speed bits and that is intended for serious track use.
However, IMHO a wing (like one of these) alone could make the nose a bit light at speed -- even for an unmodified ST. That's not what you really want when taking turns at the car's handling limit. If we had a RWD car, it would be more reasonable -- still nose and tail should have complementary bits.
For me, I opted for the cosmetic (and maybe mildly functional) mountune "splitter" and the mostly cosmetic 3D Carbon ducktail spoiler. They probably have some slight effects at speed, but not much more than the stock setup, if at all. But: They're effects should be offset by each other, leading MAYBE to a little bit of added stability, but not something I'll be counting on. I just like their looks! And don't believe they will substantially disrupt the handling of my car given my mild other changes.
YMMV.