Test offline at 25% fuel and no bombs, used .wind command to get to 30k quickly then turned it off. My offline fuel burn multiplier is set to 0.02 for easy testing purposes, 1/100th of the MA's.
As I crossed 31,070ft my climb rate was 485fpm.
At 32,500ft my climb rate was 102fpm.
At 32,750ft my climb rate was 45fpm. At this point I decided to dump all ammo. I emptied the nose gun first and the recoil pushed me into a 90fpm decent. Using the top turret I aimed slightly to the side and down causing the 20mm as well as the starboard waist gun and the tail gun to all fire, spiking my climb rate to just over 500fpm until the ammo ran out. The port waist gun, aimed down and back, produced a climb of 90fpm while it lasted.
Empty of ammo and at 25% fuel my climb rate was 7fpm at 33,100ft. It took me 17 minutes to get from 31,070ft to 33,100ft, including using my ammo as thrust.