Getting back to the original question, and at the expense of stating the obvious, the X Hots are one degree "stronger" than the Miura's, AND the lie angle is different AND the weight is different AND the shafts are likely different in their flex (even if both are R), or kickpoint, etc. The process of making the head is way down on the scale of how the clubs play for you.
I would have to guess that the "distance loss" you experienced may be the differences in the shafts of the two sets. Each time I have been fit for clubs, the brand or type of the head mattered relatively little. Once they have dialed me into the shaft that works for me, I hit all of them virtually similar. There were some subtle differences, but I would have to imagine that a 10 yard difference in irons is more likely the result of whatever shafts were on them and the fact that they were game improvement rather than some difference between cast v. forge.
There are so many differences in the flex, kickpoint, weight, etc. of shafts (and yes, FEEL), that is the place to start paying attention, imho. Get fit for the clubs, then get the head you like the looks and feel of.