Is chloroplasts per cell indicative of the photosynthetic power of a plant?
You can say that but chloroplasts do not have uniform morphology across different species. Moreover some organisms such as red algae have chloroplasts of different origin. 

Real indicator would be lets say number of chloroplast ATP synthases and/or light harvesting photosystems; these can be perhaps indirectly approximated by the total surface area of the thylakoids. 

But for all practical purposes and comparison between not so distant species density of chloroplasts is a decent enough approximate for photosynthetic power. 

About turning carbon dioxide to oxygen:

You should note that the source of oxygen is not CO2. Oxygen comes from photolysis of water; this is the initial step in the electron transport chain (ETC) in chloroplasts. The ETC results in production of ATP and NADPH; the latter is used in the Calvin-Benson cycle to fix CO2 to sugar. 

So photosynthesis and carbon fixation, though associated in chloroplasts are basically different biochemical processes. Theoretically as long as NADPH is provided a plant should be able to fix CO2 [this is just my guess].

It is difficult to observe internal structure of chloroplasts with normal bright-field microscope but you can count them easily at 40x magnification. See here.