From the Gun Turret's wiki page:
"[...]Physical projectile damage (research) upgrades both the base damage of the ammunition and the damage of the turret as two separate, multiplicative bonuses, quickly leading to a much higher damage output than the SMG."
Right now at Physical projectile damage 12 my gun turrets deal 5+46.2 damage with standard quality yellow ammo, however the Heavy gun turret added by this mod deals the SMG equivalent damage which is 5+11, making it direct downgrade from the Gun Turret, despite the increased rate of fire and range
Not sure if this is intended or not, but I'm assuming it isn't so I'll post this with the bugs tag
