We have available some scholarship merging both approaches concerning this topic from the IEEE. Here is a comparison of both theoretical and measured values of vertical (left) and tilted (right) whips... B. A. Austin and K. P. Murray, "The application of characteristic-mode techniques to vehicle-mounted NVIS antennas," in IEEE Antennas and Propagation Magazine, vol. 40, no. 1, pp. 7-21, Feb. 1998, doi: 10.1109/74.667319. Further discussion is here. There's lots more to this paper beyond the above and a good investment to anyone considering mobile HF ops. This is a good question. It does seem the above research suggests it won't help unfortunately.