Maybe someone can tell me if I am missing something.
I did the math for how much extra damage you do with last whisper, and I kept getting fluctuations around 14% extra damage regardless of how much or how little armor the opponent has. Doesn't this make the last whisper more effective against low armor champions than high armor ones?
My own calculations say that LW reaches peak efficiency at 124 armor, and falls off to either side.
Max damage is 10.7% extra.
http://codepad.org/HywAsRNY
I felt like doing math too! And my results were different than either of yours! I'm not sure why. Physical damage mitigation is A/(A+100), where A is your armor.
Let's say I'm a Caitlyn with 200 AD attacking an enemy with 30 armor. Normally each autoattack is going to deal 153.8 damage. LW decreases their effective armor to 19.5 which increases your damage to 163.4 for a 6.2% increase in damage.
Now I'm attacking a mid-armor target. Let's say... 80? He's got a chain vest or something. Normally each autoattack is going to deal 111.1 damage. LW decreases their effective armor to 52, which increases your damage to 131.6 for a 19% increase in damage.
Now a high armor target! Let's do 150. Normally each autoattack is going to deal 80 damage. LW decreases their effective armor to 97.5, which increases your damage to 101.3 for a 27% increase in damage.
Next, a ridiculously armored target. 250 seems like a decent amount. You're a fed Caitlyn now and their Rammus has a couple high end armor items. Normally each attack is going to deal 57.1 damage. LW decreases their effective armor to 162.5, which increases your damage to 76.2 for a 33% increase in damage.
So my calculations show LW increasing in effectiveness as armor increases and never losing effectiveness. You get less of an increase per point of armor as the enemy stacks it higher (the damage boost decelerates, basically), but that's because armor mitigation decreases with every point of armor gained.
Only adc who doesn't really need Last Whisper is vayne.
I hope you're not skimping on armor pen on Vayne because of her W. I know nobody likes to talk about it, but she still deals
vastly more physical damage than true damage.