Here's a quick and dirty look at her DPS with the machine pistol.

The problem with Hack increasing DPS is that you have to spend time Hacking, which decreases overall DPS. Without a damage boost, you're leaving your target alive for an additional 0.8s, which is the opposite of what you want. So in order for a damage boost to matter, it has to compensate for that.

What the spreadsheet shows is that the bonuses you'd need to apply to compensate for the time spent hacking are HUGE for low HP targets. In order to kill a 200 HP target in the same time, you'd need to increase damage by +160% to kill at the same time. That percentage gets lower as targets get hardier:

150 HP targets would need a +400% damage boost to kill in the same time as without using Hack;

200 HP targets would need a +160% damage boost;

250 HP targets would need a +100% damage boost;

300 HP targets would need a +72% damage boost;

And beyond that, Sombra probably shouldn't be trying to 100-0 them.

What this means is that the targets Sombra wants to be targeting (low health heroes) would require Hack to apply a % damage boost so huge as to be ludicrous to make Hacking for a damage boost worth it. You could incorporate a scaling damage buff based on max HP I guess, but that would be pretty obtuse and unintuitive. A reasonable flat damage boost (say 50%) would be useful, but its only real use would be on tanks - in any other case, using Hack rather than shooting would result in a damage loss.

tl;dr - Having Hack apply a % damage boost isn't as good as it sounds because reasons.