I've got a friend who owes me money that has a carbon fiber fully adjustable rear wing. Is anyone running one of these and you getting to high enough speeds on track days that it actually matters?

Looks like this:
http://www.ebay.com/itm/UNIVERSAL-57...ht_2306wt_1040

 

https://www.miataturbo.net/race-prep-75/

 

a real APR wing from 949 is cheaper...

 

Without proper front aero and whatnots, a wing is a waste of time. That one is too low to be functional on a Miata anyway. I'd get the money he owes you.

 

Too expensive for what it does..

 

If he only owes you like $50 and is offering up the wing go for it, but you are gonna need at least 11 inches from the trunk lid to the bottom of the wing to get good use out of it. Also like Doppelgänger said it wont help unless you have the supporting aero to go with it.