I ride mostly off road so I cant comment on the mpg.
I have a very open air box, fmf programmer, fmf Q4 slip on with stock header and 14-50 gearing.
My neighbor has 2014 wrr with a bone stock motor and 12-43 gearing which is very close to 14-50.
I weigh about 80 pounds more than my neighbor.
We went riding at old mines yesterday doing some hill climbs.
I could stay in second and get up everything I had a small run at, he ran out of power a few times trying to use second and didn't make it up some hills, he had to use first gear places I was in second.
I let him ride my bike around a field and he claimed mines easier to ride because it has a bunch more low end power...
Every time I see someone post A WR250R has more low end power stock than modded I know they didn't do something right when they modded it, I have access to a stock one I know the difference.