I like the concept. I think it should not use weapon dice. Roll 2x superiority dice instead. However, even then it's likely a fair bit better than the regular ones that add a single superirity dice to damage.
Chance to hit most enemies will fall somewhere between 50% and 75%. At 50% my suggested change would make it basically equaivalent in the long run and situationally better in some circumstances but not all. However, at 75% chacne to hit its loads better in the long run as well as having way more situations where it will be situationally useful. In fact it'd be very rare for the regular 1d8 damage abilities to come close to this one ever. It's almost a must pick IMO and definitely a must pick for archers with their naturally higher chance to hit.
Though the super good damage effectiveness of this ability may be apprpriate since damage is all that it does.
Ultimately it's not a game game breaking ability even if it does slightly more damage. It can be justified in that you are trading this damage for less utility. And ultimately it's at most 50% more damage from superiority dice which is a relatively minor buff given the percent of your total damage a day that actually comes from superiority dice. Comparing this to wise use of precision attack and the -5/+10 damage feats it will likely result in less damage than precision attack and damage feats could do anyways, I think it will be a fine addition.