Q
Qwerty
I haven't actually thought about it that much. Ive always built my own stud walls where heavy weights are involved and made them rock solid. I dare say that an architect or surveyor may have facts and figures
Could you cut the old board out and put tile backer board in instead? Sometimes it is worth the extra hassle for piece of mind. As you say you are 3kg over, which to be blunt is still over. Hardiebacker or aquapanel would suffice.
The best way to sort out paramount board is to take the single sheet of plasterboard down, hack the cardboard squares off with the back edge of a handsaw- this sounds demented but it really works.
Then use building adhesive (gripfill, pink grip etc, solvent based) to bond CLS timber on its side to the plasterboard you are left with. Then the following day screw your board onto the CLS.
This also give you the opportunity to get the wall flat by bonding the CLS on perfectly Plumb and flat.
Prep work like this is fairly time consuming but it means you are starting the job on the perfect wall. Starting a job on a compromise often means the finished product is also a compromise.