The ISCV will NOT affect the mpg, but does control the idle speed, it is located on top of the throttle housing, and has a two pin connector plug going to it.
Clean by spraying with carb cleaner liberally, then spray with silicone spray or silicone polish afterwards.
Most likely is the injectors have gummed up, very common with low use cars that sit for ages.
If going stage three you will have to change the std yellows/blues for either greens, normally reffered to as 803's, or siemens injectors depending on the chip etc that you use, so again I wouldn't worry till then, though injector cleaner will probably help a LITTLE, and is cheap in the meantime.
tabetha