Chapter 6, Grand Prize Snow Owl set.
If I've done my math correctly, the full set produces for me 5,820 Elixir / day. A Chapter 6 Level 15 Elixir Manufactory can produce 2,487 Elixir / day (9-3-3-3-3-3). 234% output.
That 20 square Elixir Manufactory needs 2,087 Population, 417 Culture, and a boatload of associated roads, for an effective 162 squares. For equivalent daily output to a single Snow Owl set, that totals 379.08 used squares, using 4,883.58 Population and 975.78 Culture.
On the other hand, the 30 square set provides 920 Population, 1,220 Culture, 69,000 Gold and 3 Knowledge / day. Not sure how to count the Knowledge, but counting the Pop and Culture, it has an effective -72.81 used squares. If you were to replace 2.34 existing Ch6-EM-15s and supporting buildings with a Snow Owl set, that would be the equivalent of gaining 451.89 unused squares, or just over 18 Expansions.
And I thought Traveling Merchant IIIs were the bees knees (well, after Trading Outposts, which have actual bees knees). I am actually stunned.