Fuel Economy W/ Performance Mods

[AutoXBot]

Hey guys, I just recently bought a 2009 4 dr. Yaris hatch with automatic transmission. I'm really impressed with the MPG I am getting with the car, BUT performance wise not so much. I wanted to find out if adding the following to my Yaris would drastically improve or hurt my MPG:

-NST Under drive crank pulley

-Aftermarket exhaust (looking towards Tanabe or Nitto, which one is better?) Also will the aftermarket exhaust drag if I were to lower with some NF210 on 16'' wheels?

-Megan Racing Headers

Do any (by itself or combination) of the above affect my MPG? Right now everything is stock and only have 900 miles on the engine and have been getting 38 mpg. I mostly drive highway and don't want the MPG to lower. I want an increase in performance (mainly the 0-40 mph acceleration), and most importantly increase or maintain my current MPG. Any help is greatly appreciated. Thanks again for all your help/knowledge.