As I mentioned in the comments, you're looking at the wrong aspect of armor. You shouldn't be looking at the percentage it reduces, but rather the amount of damage it actually prevents as a measure of how much longer that armor lets your survive.
Let's consider an example for a champion with 0 armor. Say he has 1000 health, just 'cause that's a nice round number. Now our champion gets caught in Garen's judgement, and starts taking 100 physical damage each second (at one hit per second). Clearly, our protagonist can take ten hits before dying.
But now, imagine our champion has the 50 armor you mention, meaning the incoming physical damage is reduced. Now (still with 1000 health) it takes 15 seconds for that bastard Garen to claim our champion's life. 50 armor extended our life by 5 seconds. (100 * (2/3) = 66.6 DPS; 1000 / 66.66... = 15 hits)
So our champion respawns, wises up, and buys 50 more armor. Now with 100 armor, we're sitting at a flat 50% damage reduction. Again we run into Garen, who's still dealing 100 base damage per second. It now takes him twice as long to kill us -- 20 seconds. Once again the armor has extended our life, again by 5 seconds. (100 * .5 = 50 DPS; 1000 / 50 = 20 hits)
Back in base, we buy another 50 armor. We're now at 150 armor, and 60% damage reduction. Enter Garen -- spinspinspinspinspinspin and it takes 25 seconds before our champion dies. Again, adding 50 armor extended our life by 5 seconds. (100 * .4 = 40 DPS; 1000 / 40 = 25 hits)
So even while the amount of damage reduced per point of armor diminishes (as you observed), the amount of time armor extends your life (or effective health) remains distinctly linear.
Best Answer
The answer is: No, Wukong only gains these bonuses for visible champions within range, so stealthed champions or champions hiding inside a bush in which Wukong has no vision do not activate his passive.
Still, you could theoretically get some useful meta-information from this ability! In case you were facing an enemy Wukong :-P Wukong is a staple solotop laner, and as such he has to be very careful about the possibility of suffering a gank from the enemy jungler (which in this hypothetical situation would be your team's jungler). As a consequence, solotops tend to place a ward in a couple of critical bushes in the river in order to get some map awareness.
If you suspect that the enemy Wukong has placed a ward in one of these key bushes, you could enter them while Wukong stays relatively close (you do not want to give him an easy kill, and after all the radius of activation from his passive ability is 1400, which is quite large) and notify whether or not the status icon associated to his passive disappears (you can see it if you move the cursor over the enemy champion). If it does not disappear, that means that the enemy Wukong retains vision inside those bushes... Which means that they are probably warded.
And then you could warn your jungler not to waste his precious time waiting to gank him :-)