Assuming they don't have any bonus due to Strength (or equipment if a bow/crossbow). It sounds like a lot as a flat percentage. In the context of the game I think it matters much less.
A hard encounter for a single 5th level character is 750 XP, an owlbear is 700, or CR 3. Assuming the average damage per round, at 19 hp damage it's 3.1 rounds to kill the owlbear, vs 3.9. In other words, it's 4 rounds in either case. Obviously the higher the Dexterity, the more of an impact it could potentially have. But the game isn't one of raw math, and if combats might last a round longer, the players adjust accordingly.
Can you (or somebody better with the math than me) put together some examples of how many combats would last even an extra round because the rogue (in a party of 4) is causing 4 less damage per hit. It's also well within the math of the game. For example, the damage range for a given CR of monster is a 6 point range. A CR 5 monster is expected to deal 33-38 points of damage per round.
So I'm curious. I'm sure it becomes pretty impossible to model, since there are lots of other abilities and circumstances at play. Especially as you get to higher levels. But I think those variables minimize differences in damage output as well.