I hear that the map will be updated. I suggest that small medkits be added along the underground to aid weaker classes against stronger classes, like Scout vs. Heavy, and to weaken the pyro's dominance in the tunnels. It does not have to be saturated, but in small corners, like next to the yellow and purple elevator or in the middle of the speed boosters.
If small medkits aren't possible, add small pools of water to 'nerf' the pyro in the tunnels.