No don't install a muffler, leave the exhaust open to let the motor breathe easy and get the turbo spooling quicker. Get a GPS or borrow one and make sure that your speedometer is correct so you are getting accurate mpg. I am not a big fan of aftermarket oilable air filters but I am sure some others will chime in soon. For more power and better economy check out DP tuners.
DP-Tuner