Well, I got a partial answer:
"The amount of carbon dioxide a plant requires to grow may vary from plant to plant, but tests show that most plants will stop growing when the CO2 level decreases below 150 ppm. Even at 220 ppm, a slow-down in plant growth is significantly noticeable."
http://homeharvest.com/carbondioxideenrichment.htm

And from same link:
"SAMPLE RESULTS FROM CO2 ENRICHMENT STUDIES
BIBB LETTUCE
By adding CO2 to the atmosphere around the plant, a 40% crop increase was achieved. Whereas previous crops averaged 22 heads per basket, lettuce grown in the increased CO2 atmosphere (550 ppm) averaged 16 heads of better quality per basket.

CARNATIONS
CO2 levels to 550 ppm produced an obvious increase in yield (over 30%), but the greatest benefits were earlier flowering (up to 2 weeks) with an increased percentage of dry matter."

So, basically at the 380 ppm we are still starving the plants in our world. And historically [at least to ice core records] the poor buggers had simply starving for eons:

Keep in mind if CO2 somehow reached a global CO2 of 150 ppm plants would stop growing- which would mean mass die off despite plenty of other element needed to grow- enough water, sunlight, etc. At 220 ppm despite having all the other elements needed, their growth and ability to resist disease would be impaired.