Spells cast due to this talent behave completely equal to their base spells.
In Patch 2.3, chance of second no mana cost, half damage spell increased to 4/8/12/16/20% with no additional threat. Prior to that the chance was 1/2/3/4/5%, but spell's damage was not halved.
NOTE: After some tests its clear that at least rank 2, 8 and 9 of Lightning Bolt do full damage when proccing Lightning Overload. Rank 1 works like intended (half damage).