Here is a photo of a 20 ft+ grapefruit tree that is located in Florence SC about 30 miles north of me. The tree is loaded to the hilt and the fruit are coloring up nicely. Im not sure if its a pink or white fleshed grapefruit but the owner did tell me that he bought it from Oregon Exotics some years ago... I sure hope my grapefruit tree produces like this one. I did get a few budsticks while there!

I wonder if my potted grapefruit will get 20 feet tall and tons of fruit? Life is not that easy!

Well, it COULD be similar to the cold hardy grapefruit Kinkoji, or perhaps related to it, or maybe still a different hybrid. Since I haven't seen Kinkoji fruits at the same resolution as the pics, I could not compare. Are these in anyway similar to Kinkoji?

I can definitely see a blue canvas from the ground perhaps hanging to some hooks up in the wall, in the back, which when covered over the grapefruit during the winter, can provide good protection during the very frosty cold nights. Plus the grapefruit is near a wall and so there's a lot of radiative heating during the winter months. Planting next to the wall of the house, or even a brick or concrete wall fence help provide good microclimate for subtropicals during winter.

I had a quick look through Oregon Exotics catalogue from 2000.
The only grapefruits listed are Star Ruby and Oroblanco (which is a pummelo x grapefruit hybrid).
They also sold a pummelo x ichangensis cross, Shangjuan, which produces fruit that look rather like grapefruits but are very sour and very seedy. I think it would also have some form of the 'double-leaf' of ichangensis, which isn't obvious in the photo.
So my guess is Oroblanco. As far as I know, not particularly cold hardy, but it does produce good fruit in areas too cold for true grapefruits to sweeten properly.