Competition and welfare - empirical evidence
Here is a fact sheet by the OECD about the effects of competition. It is mostly concerned with growth and productivity, but also discusses the effects on poor consumers and prices. It cites a wide range of empirical studies that you might find of interest. 

https://www.oecd.org/daf/competition/2014-competition-factsheet-for-print-en.pdf

OECD, Factsheet on how competition policy affects macro-economic outcomes, 2014.