Not IK, but in my past life I had this done to quite a few pairs of pumps and boots. It's true that in most cases, 1 - 1.5 cm is all that can be removed without disturbing the balance of the shoe.
I take it you mean the r+b Newbury booties? 3/8" off those would already be a lot, structurally. Perhaps you meant something else though?
What *can* be done in some cases is to also add a small platform to the front. I had this done on one pair of boots and the results were satisfactory. Meaning I was able to wear them even when I hadn't kept up with my abs work so my back wasn't at its best.