Relatively new article in the National Academy of Sciences journal. Farris, Kelly, Cresswell and Lichtwark I believe have written a wonderful paper clarifying the role of the intrinsic muscles of the foot.
Farris, D. J. et al. (2019) ‘The functional importance of human foot muscles for bipedal locomotion’, Proceedings of the National Academy of Sciences, p. 201812820. doi: 10.1073/pnas.1812820116.
The conclusion has an especially important comment: "In conclusion, we have shown that the PIMs (Plantar Intrinsic Muscles) actively contribute to stiffening of the MTP joint in late stance during walking and running, to assist propulsive push-off, and that the windlass mechanism cannot support this function without them."
It now appears likely then that strengthening the intrinsic muscles of the foot would indeed improve the power and stability of the MTPJs at propulsion. Not only that, it would also appear likely that their strengthening will reduce loads on the plantar fascia during propulsion.
We now see that we should not ignore the small stabilisers of all joints in assisting the prime movers, whether that be the shoulder, back, hip or foot.