I live in Hawaii, so my young trees get a lot of high intensity light from sun rise to sunset..... And, well, I want to give them more by positioning reflective material like Mylar/Foylon under/around them.

Would this be beneficial or would it be overkill?

Ecologically speaking, young trees are designed to be efficient under low light conditions, so I don't think it would really help them. Besides, species like citruses has lower light saturation point for photosynthesis. Young paw-paw seedlings could die from extreme light exposure as an example.

In order to achieve what you want, it is the duration that you have to manipulate, not the intensity. So if you can capture excess sunlight and release it during the low light hours, then your plant would benefit the most.

There are species that would benefit from added reflected light such as from mylar foil, among them are strawberries, corn, sorghum... but not citruses.

But if your trees are shaded, it would benefit them if you have added reflected light.

I have planted my trees ("Chico" Sapodilla and Gold Nugget Mandarin) in full sun. Sun rise to sun set. Mountains to the east and west probably decrease duration by an hour total if that.

My goal would simply be to redirect sunrise light to the west-facing side of the trees and vice versa with sunset light just to increase duration that all sides of the trees see light.

My Sapodilla tree is cloned and was cut from a branch and put into the ground right away a year ago. It's responding incredibly, but my goal is to keep it bushy rather than have it grow upwards. Along with topping, would mylar/foylar help in this regard?

High light would tend to make the canopy spread laterally, so it should help. Make sure you allow winds to sway your trees so that they will have stouter trunks.

Actually Hawaii, would have low light, due to the fact that it located at sea level. Colorado would have a high light intensity due the average altitude for the state at 6800 feet. Therefore, the sun's rays have a lot less atmosphere to penetrate in Colorado. People sunburn very easily and quickly in Colorado. Citrus uses light to manufacture carbohydrates (sugars) by the process of photosynthesis. This process maxes out at 650 PAR which is about 1/3 sunlight. So any additional light intensity provided by the Mylar would be wasted. As Joe stated, additional intensity would not be of benefit, rather longer duration of light would be a benefit to the tree. This could be obtained by artificial lighting the tree after sundown. - Millet