Well, being a theretical physicist (I hate throwing around credentials on forums, I know they mean very little, just to add context...), I certtainly know that there is very often a difference between theory and practice. In thery, an object in a curve does not loose any energy as long as there is only a latteral force acting upon it. So it does not require any additional power to maintain speed. Now in practice, there might be some slight energy loss from friction in the differential or whatever. I don't know since I can barely tell a wrench from a screw driver. But I suspect that the energy loss (and thus additional power required) would be too insignificant to even be relevant to this discussion.