The boost will be minimal.
Consider the following hypothesis with maximums:
* G12 pieces add +120 as max per piece.
If you had all 6 with that +120 (no toon has this, but serves for the purpose of hypothesis) it will add up +720
* A toon with 4x offense% main stat mods will add 8.5% *4 = 34% adding an offense set of 15% makes 49%
720*49%= 352.8 rounding to 352
So +352 on non crit and +528 on crit added to whatever final damage the toon already have. This is quite negligible add to final damage without the introduction of those linear offense in mod calculations.
And bear into mind that no toon has those 6 * 120 gear pieces, they have way less than +720 linear from g12 pieces