This actually happened with Twitter in 2018: they posted a narrow profit in Q1'2018 and just made it into the S&P 500. Market cap was about half of Tesla's, and they didn't have nearly as many shorts.
The outcome: over several weeks there was an almost uninterrupted rise from ~$30 to ~$46: a +50% rise.
With ~20% of TSLA float short plus twice the weight of Twitter I have no idea how far TSLA would rise after a surprise S&P 500 inclusion, but if the Twitter example is comparable then I'm pretty sure it will be more than +50% from current levels.
Not advice though.
Twitter had huge Call positions months before the S&P inclusion. I think that helped to dampen the jump to only 50%.
For Tesla, there are three factors to make it potentially more dramatic.
1. Larger overall short position, and each short tends to have outsized position on TSLA because they are so sure this is the best short.
2. Many TSLA longs are looking for 10~20 fold gains, so unlikely to give up shares during rally. Instead they are likely to buy more with increased buying power.
3. Q3, Q4 of 2020 is expected to be strong, 2021, 2022, 2023, 2024... are likely to be even stronger. So shorts can't just sell future Calls to limit rally and get away with it.
It would be best if the squeeze lasts several years, slow and steady.
