Circumnavigation on Kerbin-Rescaled to 2.5
Heat management, however, is probably my worst problem. Perhaps some fins will help to dissipate the heat?
(pumping fuel to the FAT-455 wings helped a bit - Thermal Mass matters!)
The cruising speed is not bad (almost Mach 5!!), it took less than half an hour from take-off to overflying the North Pole.