Well, as for "unlimited data" plans, after 5gb of tether use, you get cut off from tether service. But they figure you shouldn't be able to hit 5gb just using wifi tether, so they call it "unlimited"...
But yes, tiered plans are different... Either you have it, or you don't. You can cancel it, but it becomes a plan/price change, not a service/price change like turning on/off unlimited texting...
Root is unlimited/unlimited (and for free), you WILL see a large boost in data use any way you do it, and so will VZW.
Don't forget, wired usb tether breaks NO ToS conditions, and is free. :-$