The most obvious thing is nothing to do with heat/temperature. The rapid boil agitates the food a lot, to the point that if the food is soft, it can pretty much tear it apart. You probably don't really want disintegrated food, but smaller pieces do cook faster, so I suppose you can look at this as a rapid boil cooking faster from a certain perspective. It certainly cooks differently.
The other big thing is that rapidly boiling water will recover faster when you add food to it. It's not because the water itself is hotter, but the pot is, and the stove is too if it's electric. So you want a rapid boil to start, even if you don't need it later.
As for heat/temperature, there are differences between a full rolling boil and a slow boil, but not really what you're suggesting.
At a rolling boil, the water is mixing well enough that effectively it's all at 100°C. At a slow boil, it's really only boiling at the bottom, with a few little bubbles floating up from there, so most of the water is actually a bit below 100°C.
This difference is bigger than any effect from steam coming into contact with food; the heat capacity of water is substantially greater than that of steam, the steam isn't under pressure so it won't be above 100°C, and the food will be in contact with water more of the time than steam anyway.
Of course, if all you're doing is boiling water with a relatively small bit of food in it, it doesn't make a huge difference if the water's a little below boiling. But if you've got a lot of food and not that much water, like in a stew, the difference can become way more pronounced. Convection becomes inefficient, so at a simmer or a low boil the heat doesn't propagate from the bottom to the top very efficiently. That lets the temperature at the top be significantly lower, and so things will cook more slowly. Covering the pot does mostly mitigate this, if it's an option.
Finally, the bottom of the pan is substantially hotter than the water, and if you've turned the stove up higher to make it boil faster, it'll be even hotter, so food that comes into contact with it will cook (or more likely, scorch) faster. That's not directly due to the faster boil, of course, just the heat being transferred from the stove to the pot, but they go hand-in-hand.
So yes, things do sometimes cook faster at a rolling boil (what you call "boiling it really hard") than at a slow boil, but it's not because of steam coming into contact with the food, and once it's truly boiling, adding even more heat doesn't really change anything in terms of heat.
Your dealing with a few things here. First is starches in general. The thing to know about starches is how they gelatinize and at what temperatures.
This powerpoint is a nice primer on that topic. www.cfs.purdue.edu/class/f&n630/gelatinization.ppt
Basically your dealing with amylose and amylopectin, together they are what we know as a starch. When they come in contact with water the starch cells begin to swell and when their gelatinization temperature is met they burst and release their contents into the medium they are in. In the case of a dough ball, your dealing with tons and tons of little cells being held together loosely at first by the physical pressure of kneading them into balls and then when the heat causes the starches to gelatinize they adhere to one another kind of like being caught in a net.
Now the fluffiness portion of the question can really depend on how the dough is being cooked. For something like a dumpling being cooked in the boiling water, one would typically want to work the dough as little as possible to avoid making it too dense and if using flour to lessen the gluten formation that can make it very chewy like bread. Some recipes call for leaveners that can create gas bubbles when heated to a certain temperature and then through gelatinization the bubbles are trapped inside the dough and create an airy texture.
Best Answer
I can think of two potential things that might affect how delicious food is:
If you're the one cooking, constantly watching the food makes sure you don't burn it or do something else that might adversely affect it.
For everyone involved, cooking or not, it can help to build anticipation. You'll smell the food cooking, and may trigger physiological changes (stomach rumbling, anticipatory salivation, etc.) as your body prepares for food.
As your body has an opportunity to start thinking about the food and there's a period of denial (as the food hasn't been served yet), the food may seem more delicious than food that's only set down before you just as you're preparing to eat. (Although, if the food quality is lacking and doesn't match what you've been anticipating, it's possible that your enjoyment would be worse)
To know if it's truly eyesight that's required, you would probably have to do some experiments -- watching someone cook behind a glass window or via closed circuit TV so you can't smell it being cooked; being blindfolded so you're in the same place to get the smells but can't actually see the food being cooked, etc.