I've noticed a decrease in fuel mileage with my mods (piston, pipe, cam, etc), but it only makes sense, your opening up the engine more allowing it to breath better u know? Honestly though the difference is minimal.
But man, if your worried about gas mileage your in the wrong sport.
Well yeah, as a rule of thumb if you have to rejet for a mod it's going to use more gas. We're talking about maybe a 5-10% loss in fuel mileage though, it probably won't be noticable unless you're obsessive compulsive or something and log your fuel usage and mileage.
With my mods i know i'm using more gas than i did when it was stock, but i can still trail ride with some hard riding mixed in and go all day on the same tank. But riding all-out for extended periods really eats up the gas, at the track I empty a tank in less than 2 hours. With all the other costs involved in this sport, fuel mileage should be the least of your concerns.
I never really noticed it, i do use more but its not like i check it, when my dad runs out on his dirtbike im just about on reserve so it works out well.
Everything else i ride with has a smaller fuel tank except for the utes, like a 300ex and etc, ill still be riding when they arent so i never worry.