The additional 50L easily translates to more than 2.5 hours of added endurance. I haven't looked up the figures but the 'traditional' overloaded configurations do not appear to be necessary if you use an engine with a low enough fuel burn. The C172 that Travis Ludlow used burns around 19.4L/hour at 70% power (TAE-125 engine), equating to just over 9 hours endurance (178L usable).
Long range tanks on the Shark UL can hold 150L, plus the additional 50L in the backseat. At medium cruise (65%) the Rotax burns 18L/hour so that is just over 11 hours of endurance at 135KT CAS. There is also a note in the AFM stating that the aircraft has been designed and tested for 600kg AUW but the manual contains limits based on the certification as a Czech UL so that is where the 472.5kg MTOW comes from. Even though you should not use a higher take off weight than the AFM limit of 472.5kg, I guess it would be safer to overload this type than a different type that has been designed for the MTOW in the books.
Last edited by Jhieminga; 2nd Feb 2022 at 18:15.
Reason: Looked up some numbers...