This plane was designed to carry crafts between its wheels such as small rockets as you can see in one of my latest posts (Launching Probe System). It was also designed to be super efficient and have a big range so you can launch the craft you carry wherever you want on droo!
Here the plane is only at 50% of its total fuel capacity and can easily fly around droo once, i haven't been further but maybe even twice!
For long flights you should use the afterburner (100% thrust) until 20-25 km altitude then set throttle on 95% with a pitch of 10 to 15 degrees. The plane will then slowly gain altitude and speed as it loses weight. I have reach a ceiling altitude at ~ 37km at ~ mach 1.7.
The plane is not very handy because the wings stability modifiers are at 50 wich makes the plane stable when using nav sphere, set those at 1 if you want to pilot the plane manually.