Your argument about the whole "math" thing isn't actually a valid argument.

Don't believe me?

Look, there are 4 types of encounters in this game:

-You suprise enemy (Advantage = You)
-Enemy suprises you (Advantage = Enemy)
-Both players start shooting at each other at about the same time (No advantages)
-Both players walk past each other without noticing the other (No advantages)

Obviously there are some special circumstances on top of these (height, distance, weapon type, etc.), but if we assume both players are using the same loadouts and are both on even ground, then whoever shoots first will obviously be the winner.
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Which brings me to my first point:

1. Damage mods are for aggressive fittings, extenders are for defensive fittings.

Wnat this means is that if you are good at flanking, keeping the advantage, and using intel (radar, scanners, squad chat), you will do even better with damage mods. The only thing that can counter advantages is reaction time or more ehp.

Shield extenders should be used if you are not trying to fight back as much, and instead only rely on your weapon to defend yourself if running away isn't an option.
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2. Reaction time counters damage mods

If an enemy can react faster than you can deal enough damage to defeat them, you lose. Damage mods make sure that the enemy has little time to react, increasing the chance of success.

Shield extenders are good for players with slower reaction times, and players that find themselves constantly being the one flanked instead of the one flanking.
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3. Even grounds are where extenders shine

If you constantly find yourself in 1on1 matches where both you and the enemy standoff until one player goes down, extenders are the better option. This is where the whole "math" thing comes into play. Yes, extenders will always win over damage mods if neither player has the advantage.
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To sum up:

Damage mods are only good if you can actually deal damage to your opponent. If your opponent has the advantage, your damage mod will have been wasted. When you run damage mods you must always take the advantage to make use of the small decrease in time to kill.

Shield extenders are better for making sure you have enough time to find who is shooting at you so you can fire back. They also provide a good buffer for armor tanks as the longer you can delay the enemy eating into your armor, the less time you have to spend repping that chunk of armor.

Either way, it all depends on what kind of fitting you're using, your playstyle, your reaction time, and especially your weapon. Shotguns can OHKO without damage mods, but damage mods just increase the damage that the shot does in case you miss. The forge gun is completely different though, as damage mods can make the weapon destroy turrets in 2 shots instead of 3 or 4. This not only decreases time spent shooting/reloading, but conserves a lot more ammo too.