Can't say I've noticed a huge difference with people in the back, but admittedly having people in the back is unusual for me.
I've experienced both. The only time I noticed any adverse movement is when the passengers in the back start dancing. The car will move around as if dancing itself. But crosswind wise... my VW Beetle was worse with 2 or 4 passengers.
I put my tire pressure at around 38, the dealer inflates at 32. I am using the same tactics as the gentleman above where you pwr to 40mpg and release to cruise on EV at 35mph I get around 67MPG on flat roads with this technique. On Freeway setting up the cruise at 60-65 depending on traffic gets me easy 50-53mpg, if I dont use the cruise sometimes I get 47-49. I love the savings. $250 dollar a month in fuel savings!
My first car after I moved to New York was a 1968 VW Beetle, but it was strictly stock (and faded blue from having lived by the ocean all its life till we bought it). Here's a colour-corrected version of your pic:
Spent $12.5K on the car over the acquisition cost. Not much of the original was left. webers/shocks/disk brakes/sports seats/body kit/big bore engine...............