"Is this feasible?"

It can certainly be done. Whether it's something you should do will depend on how much effort you are willing to give it and whether simply buying a new tree would be a better option. The good news is that mangos are very easy to transplant compared to many other types of trees.

"if so, any tips?"

First, as to the size of the rootball you'll need to dig, here is a generic guide, but I think it will give you a fairly accurate idea. With more sandy soil, you may want to go a little deeper than what you'll calculate from the guide.

Generic Rootball Size Guide

"My plan is to prune it back pretty good..."

Pruning back at transplant robs the tree of needed energy reserves, removes potential to make more food, and redirects energy away from growing roots and into healing wounds. It also reduces hormone levels that would normally tell the tree to grow more roots. IF you don't get a big enough rootball, removing foliage (not wood) can keep an otherwise critical tree alive in some situations. HOWEVER, if the rootball is of sufficient size, removing foliage is not necessary. In either case, cutting a tree back at transplant decreases the tree's ability to quickly establish in its new location. If you think there is a reason that your tree will not have a sufficient root system to support evaporation immediately after transplant (if you are unable to get a normal sized rootball, your rootball falls apart and you think many roots are compromised, etc), then try removing foliage. Cutting the top half off of leaves is one way to do that.

"...and apply some superthrive"

You could also dance around the tree three times on one leg. That might do about as much good. SUPERthrive won't hurt your tree, but I doubt it will do much to help, either. Few professional consider the product beneficial (or at least cost effective). Use it if you want, but certainly don't believe it is a necessity.

"Will it be okay if I move it this winter or should I wait for spring? It is growing super fast this year so I'm thinking the sooner the better."

I agree with your reasoning. I don't think the season is going to make much difference in your situation AND, like you said, it's getting bigger.

Here is a guide that I think may provide you with lots of helpful info. If you have more questions (especially about the information in the links), feel free to email me.

Planting a Tree or Shrub

"I live in hawaii at sea level and have very sandy soil."

Just for curiosity, what island are you on? I have family in Lihue and do down every once in a while. I often think it would be neat to live there with all the tropical stuff you can grow, but I guess it's one of those grass-is-always-greener-on-the-other-side-of-the-fence type of things. We grow a whole lot here that wouldn't survive there. Maybe if I win the lottery, I can get a vacation home there. LOL

Brandon- thanks so much for the help. I live on Oahu. It's great! I couldn't imagine living back on the mainland now. Sometime when u come to Maui you'll probably end up staying:)

I tried to follow the directions, but had trouble getting the root ball to stay together. My soil is very sandy so it just fell right off the roots. I'll probably do what u said and cut off some leaves to help it from drying out. I'm feeling pretty pessimistic about it surviving, but I'll keep the thread updated so others can learn from my success or failure!

If anyone has any words of wisdom, please share!

Just be sure to keep it watered well. Hopefully the roots weren't allowed to dry out before getting it back in earth/sand. It would have been nice for at least some of the rootball to stay intact, but at least you got a good amount of roots. If the roots didn't dry out too much, I'd say it has a decent chance.