The reservoir level rises as the clutch wears. Excess oil will eventually cause the clutch to slip.
If a worn clutch slips just a little, it will soon damage the pressure plate. These are about £325 from Motorworks. Probably 2x that from BMW. Mine was a little over 50% worn at 50K miles. Maybe I should have replaced the friction plate while it was apart but I'd already spent a fortune on powder coating the frame, suspension bearings and having the U/Js and final drive rebuilt.
Motorworks prices will be about £400 to £450 for a complete clutch (pressure and friction plates). If the pressure plate is ok you'll be spending about £90 to £135 for the friction plate.