It's the rapid decel that is causing the damage. If the recovered power is greater than the losses in the system and the servo driver, the difference will end up being dumped in those capacitors ie as a voltage surge. You can't store much energy in a couple of small caps like those, so you need to either limit the max decel or fit some form of voltage clamp. Think about it, all that inertia (kinetic energy) in the work, vise, table, saddle etc moving at speed has to go somewhere, ideally as heat where it won't cause a problem.
On my larger machine, I have a braking module that clamps the voltage somewhere between normal voltage and the max allowable voltage for the servo drives. If you don't have one of those, you simply need to limit your max decel rates. If you don't know what is happening with the voltage (have you got a volt meter or scope?), you will only know you've overdone the decels when you pop yet another drive. I think you know of some settings that don't work now - 20 ins per sec per sec from 300 IPM.