Same problem here, we use @2x and @4x but in some small sprites we do not need this so we use only one .png version.
We have a lot of different images and sprites so any 20-60kb are critical to be under 50 mb Google Play (avoid wifi connection alert), and also affects to User Acquisition (less size, more downloads).
I found that this warnings really affect performance in some game situations. It would be really helpful to have some kind of flag to deactivate this warnings in Corona.
Thanks,