One - the external thermostat will cycle the freezer more often, in shorter bursts. I'm not sure if it will hurt, but it may reduce the life of the freezer from ten years to six or seven - someone with more fridgie tech knowledge than myself might care to comment and correct me.
Two - if you use a vertical freezer, all those bets are off. This 10% figure is for top-opening chest freezers only. The reason it won't help with a vertical is the same reason your fridge is so inefficient - when you open the door, ALL the cold air falls out. In a chest freezer on the other hand, the cold air can't fall out and you generally only disturb the top few inches of cold air.
Now to a more vexing question in refrigeration and food - I have a fridge that has "temperature zones" designed into it to provide me space for vegetables, pickles, and so forth. But each zone is small, and I like a lot of fresh vegetables. No help here, they are confined to the crisper section, and subject to temperature fluctuations every time I open the door.
The chest freezer with the external thermostat is the answer here - you can be sure the temperatures will stay very stable at the bottom, and fluctuate a lot less than any zone in a refrigerator near the top layers. It's inconvenient, of course, to lift things out of the way when using a "freezerator", and there still aren't many areas of distinct temperature for vegetables.
So I've come up with a solution. This will not work for you if you don't have a shaded area outside, and at least a drip or buried irrigation system. And I'll explain how you can have a "mini-coolroom" that is effectively free, and operates for free. Stay tuned to the following few articles!