For improving the soft-field capabilities of your RV-8, try 380/150-15 tires. They are about an inch bigger in diameter and about an inch wider than 500-5, but they fit on the standard 5" wheels.
With a little advance planning, they fit in the standard wheel pants. I mounted my wheel pants about a half-inch higher than the plans and instructions.This caused a minor interference with the brake caliper, which I remedied with a fiberglass blister fairing.
Of course the tire clearance cut-out in the wheel pants needs to be increased appropriately.
But the bigger 380/150-15 tires make a MARKED improvement in handling on soft surfaces. You can search for it here and see much other discussion on them.
"The Magic Carpet"
Hobbs 420 in 7 years
also LS-6-15/18 sailplane