Ran into a very interesting paper researching coffee productivity (was actually searching for artocarpus response to light at the time, but I'll take it!)
http://corridor.bangor.ac.uk/documents/treesoncoffeeplantationsreport.pdfKey takeaways:
* Photosynthesis rates of sun-adapted leaves are about 80% at 500 umol/m²/s, 90% at 750 umol/m²/s, and 100% at 1300 umol/m²/s. Full sun is around 2200 umol/m²/s; the extra sunlight only serves to inhibit photosynthesis
* This is amplified by means of the fact that coffee leaves live longer in shade, and thus less energy needs to go into their production
* Opposing this, however, is real-world data showing that coffee production is higher in the sun than the shade - multi-year yields are just under 80% of max in full sun, 100% in 75% sun, a bit over 80% in 50% sun, and around 30% of max in 25% sunlight. "Good year" yields are even better for full sun - nearly as good in 100% sun as 75% sun, and nearly 4 times more in full sun than 25% sun.
* The reason for this is that the internode length increases in the shade, and thus fewer flowers are produced (note: not discussed in the paper, but if one wants to counter this, blue light is known to reduce internode distances).
* Somewhat (but not significantly) offsetting this is lower fruit drop in shade coffee.
* Because energy is abundant in shade coffee, but full sun coffee produces more fruit, shade coffee yields a higher-quality fruit, while sun coffee stresses itself out to the point of resulting in a partial alternate-bearing habit.