The reason they went to 50mm is to make a more flexible axle for heavy rubber on the euro tracks. If you don’t have that kind of traction happening at your track 40mm is an advantage in reduced bearing friction, less rotating mass and the availability of axle bearings with rubber seals.
FAA certified jet engine and aircraft technician,
Nicholson Speedway class champion 2001,
Yamaha KT100 Service Center,
41 years karting experience