I've got a plasterboard wall with indeterminate gap behind and a wide,
heavy-ish mirror I want to hang on two slotted supports fixed to outer sides of
the mirror.

Does anyone know what the weight limitations on these redidriva fixings are - I
would guess this mirror is about 40lbs in weight so I'm asking for 20 lbs per
fixing.

Additionally, the screws that are then driven into the fixing itself will need
to protrude by about 3-4 mm to catch into the slotted fixings on the mirror. Is
this acceptable?

thanks for help.