i will be short and quick: change it
even if its a good skill that can be called ultimate due to the huge attack range and vision it grants, is not what we want for xerath. What youre doing is just mixing the live versions of his ultimate and locus of power, that is only guttering the fun part of the live xerath, that is getting extra range for his skills while in locus, not only his ultimate.
the current pbe version of his ultimate seems like his basic skills and his R were designed by different people, you have a xerath when you use your basic skills, but youre a totally different champion when you use your ult, it have 0 synergy with his basic skills.
why not an ultimate that roots you but grants your skills certain buffs, like you fire your Q at max range without charging it, extra range or area for her eye of destruction, the E gains extra range and/or pass trough units stunning them, not stunning the first target it collide, and R consuming mana like swains ultimate while rooted
i .know i'm not the only one that feels that ultimate dont look like a real part of xerath, so please, change it, stop wasting time trying to tune up a thing that just dont fit in xerath's concept.