Firstly, most trees and plants are green from the chlorophyll in their leaves and stems, correct? Some trees also have red leaves, for example, where I work there are two trees with red leaves all summer (not just in autumn) in an ornamental garden.
Why are these trees red? Obviously they evolved that way (or were artificially selected for that reason), but what is causing them to be red? Is the chlorophyll red, or is it mixed with another dyeing agent to produce the red tinge? Are the trees any less efficient in their photosynthesis than purely green flora?
Secondly, how does a roosting bat stop blood flowing to its head and pooling there? Is a bat’s heart upside down with respect to the orientation of our own? Does a bat have special valves in it’s veins that stop the blood from pooling? If so, how do they work? I can’t see how valves similar to human venal valves could possibly work with a creature that spends a lot of time hung upside down.
Firstly, the red, or more commonly purple, leaves have chlorophyll in them, and it’s green. They also contain anthocyanins which vary in color from red to purple depending on the acidity of the leaf sap. If a leaf has high enough concentration of anthocyanins, their color masks the green of the chlorophyll. As far as I can tell, no one is really sure just why some leaves have a lot of anthocyanins, but it’s thought to have to do with protecting chlorophyll from damage by too much light. Even corn plants will turn purple under just the wrong growing conditions.
Your red ornamentals are likely just as photosynthetically competent as their green brethren, they’ve just been bred for their unusual color.
Secondly, I suspect that bats are small enough that their hearts can easily overcome the 4" to 1 foot pressure drop from their head to their toes, regardless of orientation. A 6 foot human encounters pressure differentials on a much larger scale.
The problem with asking about plant efficiency like that is that it’s not single measurement. Plants vary in their efficiency depending on where they are growing. Crabgrass might be highly efficient growing in full sunlight in warm weather, but it is so inefficient that it will die if grown under shade in cool weather. Many temperate forest plants will happily tolerate cool shady places, but will become so inefficient that they die if exposed to full sun on a hot summer’s day. Basically plants are machines, and the more finely tuned they are to an environment the more efficient they will be running in that environment, at the cost of lost efficiency in al other environments.
So there’s no way of knowing whether your red plants are more or less efficient than any other plant. Generally speaking we might say they would be less efficient growing in full sunlight, but then a great many green plants will die if exposed to full sunlight, so that obviously doesn’t apply universally. And if the plants are growing in even partial shade then there’s no way to even make an educated gues about relative efficiency. There are quite a few naturally red plants out there, but they are all plants of forests and adapted to deal with low light levels.