Well, I can tell you I did mess around with making thorns cover the entire arena....and ran into a major problem. It seems, after the size got about double normal, the spikes stopped going out further, but the damage continued. Worse, it became 0 cooldown, and 0 mana when this happened. Never did figure out what happened.
I can't think of a way without seriously rewriting spikes to make it stop when they hit something, other than possibly making all of the spikes stop at the same radius.
I'm not familiar with QoT from beta, so I have no idea how hard/easy/etc. this would be.
I can still try and help others troubleshoot their problems, but I'm afraid I'm out on this one:(