Is that ALTA XL big enough to swing 28" props? To swing those an X12 frame would have to be in the 1800 or 6' foot range, (assuming a 12" or so centerplate and a few inches clearance between props) , didn't quite look like it from the pic in that thread, but hard to tell. With props extended we' be talking 8'-9' span. Getting up there (no pun intended), I fly an X8 with close to a 7' span (28" props) , and it does feel, err ... fairly beastly ...
Big fan of X12's too, but I keep asking myself what their advantage would be compared to an X8 if it is swinging smaller props, as you won't get the sort of efficiency U8s are known for.