What would you do differently if trying to brew a low-carb beer? Could it be done with an extract brew?
Without an additive to dry the beer, it will be about 45-55 percent carbs. It's obviously possible to brew a light beer by starting with an OG in the 1.030's, but to get it low carb, I would suggest adding a pound of honey or another 100% fermentable sugar. That will give the yeast a second wind, and it will pull the FG down to around half what it was before the addition.
With this method, all-grain and extract will yield the same results.