>But even if the API is the same, it doesn't mean it will have the same performance? Something that was not a problem with previous GPUs could become a bottleneck and vice-versa? And since you need to support both GPUs or else you drop older iPhone support, it might get annoying.

If "in certain cases, the performance profile can be annoyingly different" is the outcome of a tectonic shift in the underlying hardware, it would be a significant and praise-worthy achievement.