My airplane has wingtip-mounted fuel tanks, installed under a supplemental type certificate (STC). In many ways, they've transformed and improved the machine by adding greater loading flexibility, thanks in part to a gross-weight increase. What drag they produce isn't noticeable, and the additional endurance means the airplane is faster over some trips than it was before. For many of my destinations, I can depart with full tanks, fly to my destination, shoot an approach, miss it and fly home with reserves.
This is a companion discussion topic for the original entry at https://www.avweb.com/flight-safety/accident-probe-test-pilot