Well they do have documented evidence of the vapor cloud that forms just at the entrance to the carburetors. It goes way back too. The spitting is called reversion. You can look it up, lots of information about it too. Basically the more aggressive the port timing or cam and valve timing you have on a engine, the more reversion you get or spitting. With more reversion the vapor cloud that forms at the entrance to the carb gets larger. With our model airplanes, zipping through the air at speeds 60mph or so greater, there is more of a tendency to blow that vapor cloud away which also affects the carburetor and how it draws fuel too. Using a short intake stack does help a lot with the spitting from reversion and helps keep the vapor cloud intact too. Thus the carb works better and the engine engine runs better. You don't normally see a performance increase, but the fuel consumption of the engine can improve some though, as less fuel is lost due to the spitting.

Since cars usually have air filter boxes and plenum chambers etc. you don't normally see the reversion effects with them. But it is still there though. A racing engine with a hot cam in it can spit a lot too.