Expected: Wearing rear-claw extensions on a felar would help with assassin kick damage. This is based on the description of the rear-claw extensions: "These were specially built for the felar chief. They seem to provide an extra row of claws to inflict twice as much damage when kicking and raking opponents."
Actual: I can't see exact numbers for damage, but based on damage noun damage doesn't seem to be affected by wearing the boots (beyond the damroll they provide). Out of 10 kicks with the extensions, I had 6 Mangles and 4 Massacres. Out of 10 kicks without the extensions, I had 5 Mangles and 5 Massacres. That definitely isn't "twice as much damage", as the description says.