2012 Cruze LT 6sp 1.4Turbo Question

I have had my 2012 Cruze since Nov of 2014. I bought it used with 32,000 miles. It just went over 55 thousand miles. It was extremely pampered and totally stock. Then, it regularly got 36mpg city and 39mpg on freeway in Los Angeles, as normally advertised. For about the past year, I have had a K&N cold air filter assembly on it, and I run the tires with 50psi. Now, I regularly get 42-43 mpg on freeway in LA, and on highway trips out of city, I get 45-46mpg. I have been considering putting on an aftermarket muffler such as a Walker Turbo, and also replace the resonator assembly with straight pipe; in hopes of improving my economy even more. Any suggestions? Thank you, Darren.