The depth of the housing on the 21.5" is less than that of the 24" and we have no way of knowing if the 4670 is the max TDP GPU the current iMac can handle because iirc the only other option at the time was the 4830 which was a weird release because it was released after the 4850..
For instance, in a 2" notebook (Clevo X7200), you can cool 330W worth of TDP between two GPUs in a CPU whereas in 1" Apple is only cooling about 60W currently. This suggests the depth of the housing rapidly becomes your limiting factor.