The i8 Spyder should be the same size as the i8 is. Just a chopped top. The Z5 could definitely be the competition if it comes with the hybrid powertrain. Not too sure how much they're expecting to put out though
If it's anything like a true successor to the Supra on Toyota end of this deal and something that sits above the Z4 and comes to life how i'm imagining it, then it won't be a direct competitor but close enough in price, power, and the like.
4C from Alfa Romeo might be the closest to this ZSX