When there was a bug that "refueled" your plane after a disco, I tested this and found altitude had a huge impact on rate of climb as well as speed.
I used to routinely fly a 163 to the tank bowl in the crater MA map, bag a plane or two, and fly back over the mountains to land.
The key is to burn as little fuel as possible to get to altitude and I never fly the 163 with more than 1/2 throttle and most often the throttle is only advanced until I see the fuel burn move just slightly above the minimum.
At high altitudes, you can fly faster than the plane can handle at this setting but at low altitude it barely stays above stall speed.
Some missions last longer than 30 minutes.