The fenders are a rules thing, D Mod also covers full bodied production cars that must retain some semblance of a car.

The problem with frame mounted fenders is that they need to be high enough to clear the tire at full compression. This puts then pretty high where they will catch air ( drag ) and may block view. By the time you make suitable mountings, the total weight will increase.

As a test you could remove the fenders run B Mod and do an A - B - A test to see if the loss if unsprung weight helps.