The FWB65 was my first 'serious' target air pistol purchased in the late 70's and although my hands are not as broad as yours (9cm across knuckles you said) I found that customising the grips by the addition of plastic padding or something similar worked successfully and cheaply! By trial and error you need to add the padding and, once set, file it down accordingly to achieve a comfortable fit which should result in repeatable hand position on grips leading to (hopefully)tighter groups and higher scores. I still own the 65 but use a FWB P44 for competition etc. Better materials than plastic padding are available now however; 'Polymorph' available from Maplins is apparently the stuff to go for although I have not tried it. This may be a cheaper solution than buying a new pistol - worth a try!
Aubrey