Don’t ask me about gas mileage. It’s a subject I never cared about or studied. As far as going quick yes an X or J head can be made to go fast, real fast. My street car ran 11.82 with them back in the 1970’s and that was with junk parts. A friend has a set running 9’s with small valves but it’s a drag only car.
Yes I have a set of x heads on my 340. 11.86 at about 3900 lbs. I built it to go 12.20's at 6000 and on my first run it went 12.20 at 6 grand. My friends were not amused.