Yes, I've pointed it out many times in the past - tanks would get significantly buffed if LW and VS were niche items and not good against anyone.
How about it penetrating only bonus Armor (Maybe 40% to compensate) - would make it terrible pick against squishies, while remaining a good item against tanks. The same should be applied to the mastery (Devastating Strikes) as well and to Void Staff.
Also, the way you did the calculation is somewhat incorrect. You should instead look on how much damage it gives you, rather than how much armor it shreds - simply because it can be cost efficient based also on your AD - not only on enemy armor. For example, against Caitlyn with 86 armor it will give you roughly +19,3% physical damage. So, taking it into account let's look at different AD values:
1: 100 AD = LW passive will give 19 AD, worth 684 gold, making LW 92,3% cost efficient.
2: 150 AD = LW passive will give 29 AD, worth 1044 gold, making LW 108% cost efficient.
3: 200 AD = LW passive will give 39 AD, worth 1404 gold, making LW 123,6% cost efficient.
4: 300 AD = LW passive will give 58 AD, worth 2088 gold, making LW 153,4% cost efficient.
Okay, and now let's look at something tankier... Maybe 250 armor, just to show you how well it counters tanks (against 250 armor LW increases your physical damage output by 33,3%):
1: 100 AD = LW passive will give 33 AD, worth 1188 gold, making LW 114,3% cost efficient.
2: 150 AD = LW passive will give 50 AD, worth 1800 gold, making LW 140,9% cost efficient.
3: 200 AD = LW passive will give 67 AD, worth 2412 gold, making LW 167,5% cost efficient.
4: 300 AD = LW passive will give 100 AD, worth 3600 gold, making LW 219,1% cost efficient.
Mind you that there is a bit of difference between actually giving AD and what LW gives - for autoattack purposes it have absolutely no difference, no matter if you crit or not, or your attack speed - buying LW will act just like buying that much AD. However for abilities and items there are some differences:
- As a general rule (some exceptions include Darius' W and Blitzcrank's E - since they have no base damage) physical damage abilities benefit more from Last Whisper than from AD.
- LW will also make some items more effective unlike AD (Botrk, Trinity Force, Bilgewater Cutlass, Muramana, Iceborn Gauntlet, Sheen).
- Abilities that scale with AD but deal physical or true damage don't benefit from Last Whisper's passive, making it less effective than buying AD (Darius' R, Kog'Maw R (I think, not sure)).
So basically - what I wrote up there is the minimal cost efficiency UNLESS you use a lot of magic damage or True Damage that scales with AD - which you probably aren't, unless you play Miss Fortune (Mind you that things like Varus W, Vayne W or Kog'Maw W aren't included here, since they don't scale with AD either).