Since they shoot so well .. I would stay with them. A new spring is not that expensive and eventually it will need replaced no matter what weight you shoot with.Too light a pellet is more of a problem than too heavy imhopost us some groups for some of us (ok me) to have something to shoot for with my tx200hc
I have been shooting both 8.4 and 10.3 out of my 124 for over twenty years. Not much difference in accuracy less than 20 yards but the 10.3's carry better past that. I can tell no difference in the shot cycle between the two.I too would think the 10.3 would be easier on the spring and if anything maybe harder on the piston seal. I have not seen any objective evidence showing that heavier pellets will damage a main spring. Everything I have read or heard is always some airgun builder advises against it. I shoot what I want, when velocity falls off rebuild it.
So far, I'm not buying the theory that heavier than normal pellets shorten the life of the spring.