I do the same with the Capri homologated with a vier gang. When I'm asked, I just answer that being poor, the use of every gear I paid for is a must! Over the years, we bought some used parts for our 911's. Surprise when we checked the real capacity of an ex-works fuel tank engraved 100 litre, it could swallow almost 120 l of fuel, filller neck included. To keep a good relashionship with our german supplier, we didn't complain about this… MF that I wont name is a good bloke.
Never heard about the 5th gear trick about the Minis. Is really racing cheating? Of course not.