This performance chart here is why I think Apple standardizing on dual GPUs will help programmers figure this mess out.
Barefeats says two is better than one, but three isn’t necessarily better than two. And sometimes, one good one is better than (or really close to) three.
Plus, those complaining of Thunderbolt adding more power, read closely for what you have to do to add multiple GPUs. In my mind, Thunderbolt seems like it would be easier.