Impact is an atrocious quality, so...no, making it more available won't cause balance problems.
Seriously, the damage from Impact is worse than the damage from the 1d6 elemental effects, and those are crappier than a straight +1 enhancement. Despite that, Impact is a +2 enhancement for some bizarre reason. You have to have a gargantuan longbow before it is comparable to a +2 enhancement bonus.
For most weapons, impact is not that good. For certain specific weapons and characters, it's actually useful.
For instance: A Medium impact nodachi does 2d8 base weapon damage (which is multiplied as normal on a critical hit or with Vital Strike), with a better critical range than a greatsword, and no penalty for using an oversized weapon; that is strictly better than a flaming/frost/etc. ability on the same weapon. Alternately, a Medium impact falcata does the same base weapon damage as a greatsword (2d6). It can also be combined with an oversized weapon, such as a Large impact bastard sword for the iconic barbarian that does 3d8 base weapon damage.
Also, don't forget the bonus on all bull rush attempts, including Shield Slam. A character with Bashing Finish, Exotic Weapon Proficiency (Falcata), Improved Critical (Falcata), Improved Shield Bash, Improved Two-Weapon Fighting, Shield Slam, Shield Master, and Two-Weapon Fighting, along with a bashing light shield and an impact falcata can do quite well in DPR (as well as shoving enemies around with Shield Slam). Even better if they can afford Power Attack/Cleave/Cleaving Finish.
Allowing an equivalent for ranged weapons such as bows would probably be OK at the same bonus equivalent. However, archery is already very strong, so an adaptive impact speed composite longbow may be a bit much for some campaigns; it might be better to allow it only on ammunition is such a case.