Hi Philip, you are good at observation! (or you must love this artwork so much... :P) It's true that it takes time to handle the soft cloth physics in this case.
Did you try playing with the distance field soft shadows instead of cascade shadows? https://docs.unrealengine.com/latest/INT/Engine/Rendering/LightingAndShadows/RayTracedDistanceFieldShadowing/index.html
Finished up the Highpoly this weekend. Took a break from it for a couple weeks to work on some soft surface sculpting, but getting back into it. Going to start the lowpoly tonight.
This looks really good. Looking forward to play it one day! What kind of soft body simulation method are you using for the tires? Is it finite element based?