Is buying a Tesla that great for CO2 footprint? No, nor is buying any new car. Over the life of the vehicle, it's pretty close to a wash vs. a similarly-sized new BMW roadster, unless you drive it an awful lot. If buying a Tesla helps Tesla make more batteries more cheaply, and make gasoline a lower-volume commodity, then there's some benefit there.

Beef is bad for CO2 for 3 reasons:

1) getting water (pumping) to the beef or the cropland to feed the beef often causes CO2 to be produced.

2) growing cattle feed is extremely CO2-intensive due to the ammonia-based fertilizers corn uses. Every stalk of corn is producing CO2 because of the way we grow corn.

3) beef's popularity and need for feed means we're repurposing rainforest and releasing eons of poorly-sequestered carbon. It's a one-time hit, and then after that the damage is minimal, but it's a big one-time hit.

If you go strictly grass-fed beef, I suppose you could argue you're not producing as much CO2, but substitute CO2-intense beef would get that much cheaper in price, negating most of the benefit.