>>When you say, "only a subset of records would benefit"....are you talking roughly 10%....or 25%.....etc. ?
>
>Well, at first, only new records would benefit of it. So, the ratio would be 0.00001 or something like that. Then, it will grow on a steady base. It is highly difficult to say at this point but this new set of data could become a large part of the table.

Is there any chance you might back-fill the older rows?

Consider this - suppose you had decided to add the column years ago. Would a high % of the rows have had a value for this column?

I can't say with certainty, but if this column would have had values had you defined it years ago, I'd say just add the column to the table and don't bother with a 1x1 relationship.