The only way you will see a difference between cas 9 and cas 10 is with benchmarking programs. The 5% hit over dual channel would also not be noticeable in day to day use but in my opinion 5% is 5% and that's a decent performance hit. Unless you need the extra RAM for something other than gaming you are probably better off sticking with what you have.
You could always sell the extra stick on ebay or Craigslist.
Alright, thanks for the help. I think I'll just have a backup on deck just in case. Thanks again.