there are many types of STi wings so not sure what you are refering to but generally any Subaru OEM wing is functional.
Now, it depends on what you are gonna do. If you are driving on street, no matter how good the wing is, it is not functional. On freeways, yes probably you'll feel stability at high speeds but you'll lose top end speed.

 

I really think that pretty much every wing on the market is functional. The question is, is how functional it is.

Wings can create either downforce or lift. Regadless which of the two they create, they create it at the expense of aerodynamic drag.

Some wings can create too much downforce and increase tire wear and induce oversteer. Others can create lift and make your can very dangerous at high speeds.

The key is, to have a wing that creates optimal downforce with minimal drag. Now, finally, there isn't one optimal downforce. Low speed use vs. high speed use must be taken into the consideration.

And last, but not least, lets say you do get lucky and find specs on some wing. It might not work for you as it did for the test vehicle, since the rest of the aerodynamic package affects the overall characteristics of the car.

That is why I recommend either getting a complete aero kit that has been tested in the wing tunnel (Zero Sports, Prova, etc) or get an adjustable GT wing and find your own "sweet spot" downforce.

I don't think I answered your question. I just gave some basic info that some people might not have known or realized.