So after many years of being unsure of the settler production mechanics, I finally tested it properly and these are my results: Food is NOT irrelevant, but neither is it converted at a good rate:
Negative-0 food: +0 from food surplus
+1 food: +1 from food surplus
+2-3 food: +2 from food surplus
+4-7 food: +3 from food surplus
+8-11 food: +4 from food surplus
+12-15 food: +5 from food surplus
+16-19 food: +6 from food surplus
+20-23 food: +7 from food surplus
+24-27 food: +8 from food surplus
+28 etc (seems it continue with a 4:1 conversion rate)
So basicly:
-If you have a big city with tons of production/low food tiles such that you can go quite high in food deficity but high production, then that's beneficial
-If you have a small city with high food tiles and can't go lower than +0 for example, then remember that the first +2 food also gives +2 production
-Then again remember that if you have a city with high food, your food -> production conversion is only 4:1.
Also note: The +production gained from food surplus does NOT receive the +50% from Collective Rule, only "real" production does.
Negative-0 food: +0 from food surplus
+1 food: +1 from food surplus
+2-3 food: +2 from food surplus
+4-7 food: +3 from food surplus
+8-11 food: +4 from food surplus
+12-15 food: +5 from food surplus
+16-19 food: +6 from food surplus
+20-23 food: +7 from food surplus
+24-27 food: +8 from food surplus
+28 etc (seems it continue with a 4:1 conversion rate)
So basicly:
-If you have a big city with tons of production/low food tiles such that you can go quite high in food deficity but high production, then that's beneficial
-If you have a small city with high food tiles and can't go lower than +0 for example, then remember that the first +2 food also gives +2 production
-Then again remember that if you have a city with high food, your food -> production conversion is only 4:1.
Also note: The +production gained from food surplus does NOT receive the +50% from Collective Rule, only "real" production does.