EA - just to expand.
I once turned up for a test in a Seneca II fitted with long range tanks. I walked away and re-booked.
Why?
The refuellers had helpfully fuelled the aircraft to the brim. We were 100kg below MTOW at 1999kg. However, on the weight and balance calculation the weight was so far forward of limits that had we had an engine failure we were going rapidly down nose first.
Yes - the certification is purely a paperwork exercise, with the aircraft placarded as such and any W&B graphs stopping at the weight, rather than extending onwards like those in U.S POH's. However - balance is another matter and needs seriously thinking about if you start putting more than 140 litres of fuel (3.5 hours) a side on board.