0.8 mph is actually quite a large difference in a series with semi spec cars and engines.
For us without access to the real data - if the car is 5 mph faster empty vs full (guestimate), and holds 40 gallons of fuel @8 lbs/gallon (320 lbs) then advantage = 5/320 = .016mph/lb. Give a 50 lb weight advantage gives 50x.016 = .8 mph.
I guestimated that speed difference empty vs full - but this was my first guess (I did not fudge to reach (.8 mph).
Larger advantage no one has brought up is smaller cockpit/improved aero package. Not sure what if anything they changed on her car. But most race car drivers are on the wiry end of the scale (not counting sedan drivers like Nascar) just to fit.
Note I'm not siding with the whiner Gordon, but 50 lbs does appear to be an advantage - but a fair one within the rules. And Penske would have done it if he thought of it.
Diva's Husband
if the average speed of a race is outside of caution is 180 MPH then .8 mph is less than 1/2 of 1% of the speed everybody is running at. That's isn't large, you'll get a better push for passing from the regular drafting, or waiting until your tires are low on rubber, or tank is low on fuel.
Notice you had to "guestimate" how much faster a car is with an empty tank than full, that is NOT real data. If you've got really real data bring it, I'd love to see it.
The cockpit is entirely inside the vehicle (except for the roll bar) and shouldn't be affecting the aero package in the slightest. No advantage there.
If 50 pounds is an advantage than chubby cheek Gordon should go on a diet. The reality is that at race speeds you're dealing with such drastic abuses of physic 50 pounds isn't going to make an appreciable difference. Yes you might get a miniscule increase to top speed, possibly another miniscule increase to fuel efficiency, you'll also get a miniscule decrease to ground force and thus a miniscule lowering of safe cornering speed. In the end you have a handful of miniscule and contradicting effect normalizing to no real effect one way or the other. There's a lot of money to be made in racing, if 50 pounds granted a useful advantage Penkse would have thought of it and switched to a bunch of jockeys with shaved heads. These people spend a lot of time in the simulator figuring out every little thing they can do for an advantage, there's no way nobody thought "what about a lighter driver".