Well, yes, I think you are missing a couple of points. The C rating of a battery actually does denominate the "maximum consistent discharge rate", not the "maximum peak discharge rate" - this is a bit higher, usually, for discharges of a couple seconds with pauses in between.
The batteries themselves have differing voltage stability. When I take a fully charged Panasonic ..CH and a Samsung 20R (or AW IMR or similar), all fully charged, and I use them in the same mod with the same atomizer coil of, say, 0.8 ohms, I will see different voltages reaching the coil. The Panasonic might be around 3.8V, the AW IMR might push 4, the Samsung is around there, too, maybe a tad higher.
If the battery is not new, the values will be worse (Li Ion batteries age from the second they are manufactured and charge cycles add to the age)
The "voltage drop" is a sum of the drop the battery itself causes because of it's inner resistance plus the drop because of resistances inside the mod.
Just stay with the AW IMR 1600. There are reasons for it.