I've got a weird feeling that something is not right here:
When I just research Modular turrets, the Damage bonus on a heavy machinegun turret says, "+100%". However, when I research "Modular turrets damage 1", the bonus changes to "+30%".
How is that counted? Shouldn't it be (100% + 100%) * (100% + 30%) - 100% = 160%?

And I think that I had seen a similar effect in my previous game on a Scattergun turret, which changed from "+60% x2" (which I guess was vanilla turret bonus) down to +30% x2 when I researched something

This is just a display issue. Normally, "Damage Bonus" in the tooltip refers to the modifier applied to the turret ammo by the turret itself. For HMG turrets it is +100%. BUT, as soon as you research +turret damage techs, tooltip will show researched bonus instead, and add something like "x1" or "x2" after the tooltip for some reason (I guess it is a leftover code they didn't bother to fix).

This is an issue with base game, I can't do anything about it. Actual damage calculation is this:
"Damage: X + Y <DMG Type>". <-- this is the tooltip for damage.
X = AM * (TB + 1), AM - ammo base damage value, TB - turret passive damage bonus
Y = X * (ADR + 1)* (TDR + 1) - X, ADR - ammo damage bonus (reserach), TDR - turret damage bonus (research)

Oh, okay, thank you for the explanation.