This goes against the whole point of a MacBook Air.
Someone who buys an MBA in the first place is presumably wanting portable productivity. They are probably not looking to do this type of high-end gaming on it.
EDIT: I agree wholeheartedly with all the posters the saying that this would be an at-home solution. I admit to overlooking that part. This I would not mind even for my MBP's crummy Radeon 6490M.
But seeing as I play older games and that my MacBook Pro already has a dGPU which handles them ok, I guess I just didn't see the justification of cost for something like this.
I speak as a MacBook Pro owner so I'm not going to make any presumptions of how MBA owners would feel about this. If this seems right for you, then by all means.
Honestly, I'd totally go for the idea of good gaming at home and good portability while travelling. Games suck battery power anyways so I find I don't tend to game when my laptop is actually being a laptop and not my desktop (I use mine for both). So for me this would be an awesome solution that gets the best of both worlds. Well except that the airs still dont' get enough storage space for me.