Overwatch – How Much of a Boost Does Nano Boost Provide in Overwatch?

I've found the following sheet here which seems to have a lot of the information you are looking for. This lists the metered amounts per damage/healing and passive over time.

It's an offshoot of the main sheet here. I've posted the table below in case the page goes off line.

Hero            50%     100%    DamagePer1%  HealingPer1% DamageAmpedPer1%
Bastion         2:30    5:00    15
D. Va (Mech)    2:10    4:20    13
D. Va (Pilot)   0:30    1:00    3
Genji           2:00    4:00    12
Hanzo           2:00    4:00    12
Junkrat         2:05    4:10    12.5
Lúcio           3:10    6:20    19           15
McCree          2:00    4:00    12
Mei             1:40    3:20    10
Mercy           1:40    3:20    10           12.5         15
Pharah          2:30    5:00    15
Reaper          2:30    5:00    15
Reinhardt       1:50    3:40    11
Roadhog         1:50    3:40    11
Soldier: 76     2:30    5:00    15
Symmetra        1:20    2:40    8
Torbjörn        2:10    4:20    13
Tracer          1:30    3:00    9
Widowmaker      1:40    3:20    10
Winston         1:50    3:40    11
Zarya           2:30    5:00    15
Zenyatta        2:45    5:30    16.5         12.5

They stack multiplicatively.

From a Reddit thread, where they did some experiments:

Orb Of Discord Stacking: Confirmed! Not only that, it stacks multiplicatively! This means that orb of discord's 50% damage increase is multiplied by the 130% damage of the boost! This means that we get a damage multiplier of: 1.3 * 1.5 = 1.95 times the damage! If the orb of discord were additive (like with multiple Mercy beams), it would only be 1.8 times the damage. That's almost DOUBLE the damage! This is pretty powerful stuff!