Hmm, i really haven't the faintest idea what's going on:
i successfully caused an in-air collision between two minecarts, one on the descending arc, one step away from landing, the other fresh off the ramp. Expectedly, the descending cart bounced off and ascended again, but _reached a completely ridiculous height while doing so_. The 'orbiter' cart was launched from 24 levels up, the 'booster' cart (timed via another minecart loop to get the moment right) from 25 levels up. Without the collision, the orbiter reached a max height of 11 zlvls, the booster of 12. The 'boosted' orbiter reached 26z, and i'm not entirely sure that was the apex of its flight path; it slammed into the map edge at that height.
What makes it even more confusing is that while there was a weight difference between orbiter (chestnut, 17kg) and booster (birch, 26 kg), the higher weight _could_ theoretically convert to an ~50% increase in momentum, but that'd be less than 25% higher speed. Exchanging the chestnut for another birchen minecart gave the expected maximum flight height of 12 zlevels (both carts same weight), and this height wasn't exceeded when i exchanged the booster for a billon minecart (booster ~14 times as heavy as orbiter, no added flight height for the orbiter).
While the whole thing is rather befuddling, having a setup that propels the minecart higher than the start locations of either route is pretty nifty. Shame it's not gonna be useful for flight with passengers or cargo lifting.