Do all plants photosynthesize with equal efficiency?
If you mean the efficiency at which plants convert light energy to chemical energy (in sugar or other reduced C compounds) then there is definitely variability between plants, both at the species and individual level. The photosynthetic efficiency WP page gives ranges of between 0.1 and 8% of total solar radiation converted to "biomass", but these values are not cited. In their review, (Zhu et al, 2008) state that the maximum maximum efﬁciency with which photosynthesis can convert solar energy into biomass is 3.5% and 4.3% for C3 and C4 plants (both observed for crops), respectively. Biomass is not a measure of energy, but perhaps they mean potential chemical energy in that biomass.

Its a pretty complicated question though. Photosynthesis is a complex process involving many interdependent chemical reactions and physical processes. All species (or even individuals) of plants have different morphology, physiology, and biochemistry that influences overall photosynthetic efficiency. In addition, they each inhabit different ecological niches and climate space. Therefore, all species or individuals would have a slightly unique maximum potential efficiency. Here are a couple other efficiency related concepts to consider:

There are 2 systems in photosynthesis that work in tandem, the light reaction and the dark reaction.  The light reaction uses light energy, pigments like chlorophyll, and a bunch of electron transport chain proteins to generate ATP and NADPH. These 2 molecules then provide the reducing power for use in the dark reaction (the Calvin Cycle). In the dark reaction, the enzyme RuBisCO joins CO2 and RuBP, creating simple reduced carbon chains which are then used to assemble sugars. This is an over-simplification, as both reactions are very complex systems with lots of biochemical machinery, but this is the basic process used to convert light energy to chemical energy. To operate at peak efficiency, the entire process must not be limited by incoming energy (light), substrate supply (CO2, RuBP), or nutrients used to construct all the needed cellular and molecular machinery (N, P, K, and many others). In reality, there are limits that vary from plant to plant, species to species. For example, a species or individual with a very reflective leaf surface converts less of the light reaching the leaf surface to chemical energy, thus lowering the output of ATP and NADPH. This reduces the chemical energy supplied to the dark reaction and limits the amount of carbon fixed by RuBisCO. So the reflective surface of the leaf, which might be advantageous to the plant in other ways, has a large impact on overall photosynthetic efficiency (compared to a less reflective leaf or leaves).

If you want to search for more information, there are a number of terms that plant physiologists and ecologists use when describing a plant's (or communities of plants) photosynthetic efficiency. Each of these is used in different ways and can have many variations in units.


Light use efficiency: The amount of carbon fixed per unit of light incident.
Water use efficiency: The amount of carbon fixed per unit of water transpired
Nutrient use efficiency: The amount of carbon fixed per unit of nutrient concentration (usually in leaves).




reference: 

Zhu, Ort, and Long 2008 "What is the maximum efﬁciency with which photosynthesis can
convert solar energy into biomass?" Current Opinion in Biotechnology.