I fear this is another feel-good initiative that claims to be able to cool a city, but has anyone impartially done the calculations and done the cost-benefit analysis?  How many million dollars does it cost to lower the temp in the heat island one degree?  Is lower cost of air conditioning worth the initial and annual cost of the gardens?

Low-water plants?  They grow much slower and thus chemical reactions in the plant (including CO2, H2O, O2 exchanges) are less.  Less oxygen is produced, but I don't think you are running out of oxygen in Denver.  The cooling effects of slow growing, low-water plants are likely much less than other plants due to less leaf area and less water evaporation.  You want to feel plants keeping an area cooler?  Go into the woods not onto a roof with a garden.

Here are a couple fun web sites about plant myths.  Read the science.

http://www.gardenmyths.com/air-purifying-plants-do-they-work/

http://www.gardenmyths.com/houseplants-increase-oxygen-levels/