Update: I had made some tests with an online printing service, and yesterday I've received my custom printed cards.
Here are a couple of pics, followed by some thoughts...

All the above are custom printed cards, bar the rightmost
Battle Lust.
Printed cards don't match perfectly colors, not a real issue, as original cards do (my two original Hawkshold
Hardened backgrounds are of two quite different shades of blue, one way more purple-ish than the other). Also, image is not as crisp as in original cards and the printer cut all the damn cards slighly off-center, so the image is shifted to the right (with different degrees of severity, and I have to admit that some hardly noticeable, if you don't look for it).

The backs (original on the left, custom printed on the right) present quite some color shift, mainly in the sky. All the image is more grayish and less blue (it is less saturated, to say stuff the proper way).
Anyway, once sleeved and in-game, you'll hardly notice it (if you're not specifically looking for hues differences), and once in hand of your opponent across the table they're not easily spottable (tested myself). Ofcourse, you can guess if the card you're drawing next is a printed one, but I don't see this as a too severe unbalance factor. Plus the first three armies have a good number of revised cards, so even knowing you're going to get one of those, it will not be of so much help.Obviously, you can sleeve command decks with opaque sleeves and fully cancel the problem.
In the end, I'm not so disappointed by the results, the cards are usable and I had already replaced them in my decks.