There should be only one reason stopping you from doing this - being able to get one! I would say that for the next few months, you could only get one from FPV, and I doubt that they will sell them to anyone but a GT owner (and, sorry to say, but rightly so!).
The closest I have seen to the GT wing is the Herrods one for the AU, but it probably doesn't fit the BAs. Or just wait a few months and get a fibreglass knock-off. Or make friends with a GT owner, buy theirs and get them to order a new one from FPV ("dropped a brick on the old one").
AU1 XR8 Auto 147.9 rwkw (approx 215 fwkw)
AU2 Wagon HAD Auto
17s, Pacemakers, 3" Intake & Pod filter (The Family Hack)
FPV-XR Club ACT