Dave, I don’t know about the RR brand axle, but just be aware that if you go to a different stiffness axle, it will change the way your kart handles. That might be good or bad, depending on where you are now…
RR make good axles, they are not more likely to bend than any other brand of axle. Axles bend because they are pretty thin wall steel and large diameter. Just like William said the handling of your kart is pretty important and the best way to keep your axle from bending is to not hit anything.