Someone has done testing and found that 4 mutiplier feats stack.

They are

Weight of the World (this feat)

Empowered Renewal

Blood Champion

Aspect - Blood Fever

Battle Roar also adds 3% but this wasn't tested.

These 4 feats, triggered at the appropriate time, along with taking the Rupture feat, will make your damage very "burst-like".

"Burstlike" damage is good in PVP for finishing off opponents before they get a chance to heal.

But the above doesn't necessarily mean that you should get all these feats, just a heads up that they stack.