Looking at them, it appears that the EF adapters go through optical gymnastics that don't really work the same way as in the Game Boy adaptation. The EF-iPhone

Looking at them, it appears that the EF adapters go through optical gymnastics that don't really work the same way as in the Game Boy adaptation. The EF-iPhone adapters project the lens' image onto ground glass and then use a macro lens to image that ground glass. The Game Boy modification actually involves removing the optics from the Game Boy camera and projecting the EF-lens' image directly onto the sensor. There's no easy way to do this with an iPhone, but it is the way that would truly allow you to use the EF lens on the iPhone sensor.