I've tested my tablets many times with games. I even flipped it over and measured the temps using a digital therometer. Easy and quick to do. Nature of the beast here.
One could attempt to add some cooling if one is concerned. One could please the ipad on a laptop cooler for example. Or, in winter, go outside (ok...not a good option unless it is nighttime). Or, turn the AC on... :)
I was messing around with NFS on my Nexus 10....and I had my ceramic heater going. That thing got so warm the GPU throttle kicked on...and framerates started dropping. I haven't noticed any throttling on the iPads, though. Keeping the device in a case makes the warming less noticeable, but you still start to get a sense that things are toasty since your hands on touching the screen.
I guess as long as it is normal and being that I'm not willing to go non-retina at this point I'm stuck with the iPad 4.