As per the title.
I have played all Mass Effect games and DLCs except Bring Down the Sky from ME1.
Does it have any repercussion in Mass Effect 3?
If you spare Balak in ME1, he will sabotage the Alliance.
Alliance 3rd Fleet -10
However, if convince him to work with you in ME3, he will contribute to the war.
Batarian Fleets +15
So the net effect is +5 to Total Military Strength.
Yes, letting the Batarian leader go offers the opportunity to increase the strength of the Batarian Fleet war asset by 15 points.
Not much of one, but there is an effect.