My X pure would not roam domestically at all either. I finally bit the bullet and unlocked it, and re-loaded a custom "Based on Stock" rom and re-flashed the modem. Now it works and roams properly. I think the modem on some (if not all) the x pures are corrupt when it comes to Sprint, and since Sprint seems to be the step-child network in others eyes, it doesent get the attention needed.
I'm a long haul trucker, so I pass through roaming areas all the time. Especially in Nebraska and Wyoming. I passed through those areas, had no signal/data/voice, ended in Portland OR, unlocked, flashed, re-loaded and passed back through, and had roaming like a normal phone. Im not 100% positive but I bet it would run roaming in Canada if I ever ventured up there.
Now it did not fix the Carrier Aggregation as far as I can tell when I'm in CA areas, but at least it works. Currently using CynogenMod on it now.