My math nerd brain had to dig into this.
Let's take a 9 week flowering plant. With a more typical 12/12 schedule that is 9x7x12 for 756 hours of light/electricity
If we go with 18 hours light for the last 2 weeks we end up paying for 7x7x12 plus 2x7x18 for 840 hours, a difference of 84 hours.
84 is 11.1% of the original 756 hours.
Conclusion, if all goes according just right with a 9 week flowering plant, a yield increase of 20% may be possible for an 11.1% increase in flowering light-hours.
If you are a big commercial operation and could dial everything in this could be significant. Honestly doesn't seem worth it for a home grower.
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Shit. It just clicked. That's only part of the picture. I want this to be useful, so here comes more math.
Let's add in an 8 week veg period at 18/6.
8x7x18 = 1008 hours
1008+756 = 1764
Our extra 84 hours of light is now only 84/1764 or a 4.8% increase in the light requirements for the entire grow.
I still won't be bothering, but again, economy of scale could make it worthwhile.