Good day
From your latest post I have the impression that you are low on flow to the arm cylinder, do you have full engine revolutions. If you have a hydraulic schematic for your machine, that would help to pinpoint the cause. I believe that Kobelco rely on the main spool to control arm speed-flow both for feed and return, it could be that a second pump is not activating to help with arm in cylinder out. Measuring the speed of the arm can be very helpful.
Kind regards
Uffex