Like you pointed out, the damage is still half what the rogue has. In a bounded accuracy system, where hit/AC don't scale much if at all, your scaling has to come from damage and special abilities. If +3d8 single target at level 5 is causing problems, then they've done something wrong with the scaling of the game.
I also said I already thought the rogue sneak attack damage aw overpowered for how easy it is to get. And we're talking at will here. Let's look at our pregen dwarf. With a +6 to hit and average die rolls, our dwarf will reliably hit anything with a 16 or lower AC and deal 20 damage per round. Let's look at our bestiary, the level 5 fighter could pretty much solo anything in the bestiary at will and without breaking a sweat. I'm all for making the fighter the best at fighting, but even the toughest creature in there goes down in 3-4 rounds just from the fighter. To me that seems a bit much, though I do think it's reasonable as an infrequent thing. That's why I suggest something like burning every die spent past the first on a given maneuver, although that might require even more dice. Hmmm, I guess I'll have to run these through some play testing, see of something jumps out.