I just arrived at about 1300 HP:
Player A has x HP and a damage mod, player B has x + 66 hp and no damage mod.
DPS before damage mod is 400, damage mod adds 5%.
Player A dies after x/400 seconds. Player B dies after (x+66)/420 seconds.
The point of equilibrium is where both TTKs are identical. By the powers of linear algebra I conclude:
x = 66 * 400/20 = 1320 hp

In my scenario it doesn't depend on the base DPS.

This is because player A has the damage mod when player B has the shield extender instead, or vice versa. While TTK will change with changing base-DPS, the point where 66 hp prolongs your TTK more than 5% of damage shortens it doesn't.

The damage mod shortens TTK by a factor of 1-1/1.05, a shield extender increases it by a factor of 66/baseHP. The math then tells you at which baseHP-value these two factors are identical, which is a baseHP value of 1320 HP. In other words this is (approximately) the point where 66 additional HP make 5% more eHP for the suit.

I always just think of it in terms of the percentage of damage increase versus the percentage of HP increase. So for a 660HP suit a damage mod gives you only half the benefit of a shield as it's a 5% damage increase versus 10% shield increase. But for a 1320 HP suit the mod gives you the same 5% benefit as the shield, i.e. equilibrium is reached.

That is the same result your equation gave, but I think your equation is giving the right result (in that one case) for the wrong reason. If you plug the old values for damage mods into your equation you get 66 * 400/10=2640, i.e. you would need an impossible amount of HP for an old damage mod to be viable. Clearly, there should not be less reason to use a damage mod where their effectiveness is doubled