We have 46% efficient solar cells that are not currently cost effective. 45% of sunlight is in the useful wavelengths for plants, but the important comparison is their peak effecency with the optimum wavelength of 30% vs there theoretical limit with sunlight of 11% which is a ~3x boost under optimum conditions. Another consideration is leaf peak obsorbtion is around 10,000 lux though trees for example capture light through multiple layers. So, some plants are going to be more efferent at lower light levels. (this is also why trees don't look like a mushroom with one big leaf for maximum light capture.)

So, ignoring costs you can come surprisingly close to break even in terms of energy budget. Though we are likely better off going with GMO plants to capture more light than build millions of acres of solar farms just to break even.