kraky,
Yes, you are over-thinking the "deflectibility" of plastic-tipped bullets. I have seen a 250-grain Nosler Partition from a .338 Winchester Magnum (muzzle velocity 2650 fps) deflect so completely on a tiny twig, at most the diamater of a soda-straw, that it tipped completely sideways before hitting an eland. And not only was there a perfect silhouette of the bullet through the hide, but the twig had been so close to the eland at the shot that swirls were visible in the very short hair from the twig's branches being slapped against the eland by the bullet.
That was a soft-nosed spitzer, but Craig Boddington also had a big round-nosed solid (as I remember a 500-grain .470) deflected by a twig the size of his finger. He'd aimed at a Cape buffalo's shoulder at typical very close range while hunting in thick thornbush, and at the shot the bull collapsed, dead. But the bullet had not hit the shoulder. Instead it hit the neck, sideways, a matter of at least three feet of deflection across a few yards, and happened to break the spine.
If you want to run some tests, trying to prove your hypothesis that plastic-tipped bullets will deflect more when hitting something in front of an animal, I would suggest also shooting some heavy softpoints and even blunt-nosed solids. But based on far more examples than the two above, I suspect the results won't confirm your hypothesis.
