I suppose the op can try sprint and see. It's just a pain generally to switch.
Usually,since MVNOs ride on the same physical network, performance is same. A real sprint phone can and does roam and virgin cannot roam. But if virgin works at all, it means sprint native coverage. I guess it would be technically possible for the carrier to notice this traffic is a real sprint customer and this is an MVNO but I haven't ever witnessed that.
PS: I do totally agree that there can be, and in my experience always is, dramatic speed difference handset vs. tether. My experience is tether always faster or data-only device is always faster.
Eugene, OR -- Pacific Northwest