Assuming the amount of tradable shares for each stock is proportional to the free float capitalization (which is not a particularly good assumption), each stock in the index should be pushed up by the same amount, since the weight of each stock in the index is proportional to the market cap of the stock. I don't think this is actually true (i.e the number of shares on the market is not proportional to the market cap), which would imply that some share prices would be affected more than other. Do higher weight stocks in spy get pushed up more? I don't know. But it is certainly possible.
With respect to the stocks just outside the index, I think you could argue that they are probably undervalued and thus should offer better returns. This is what an academic would say. However, the actual reality could be a lot different: if returns are dominated by flows instead of fundamentals (like they are now), maybe going with the crowd is the best investment strategy. Or maybe not, I haven't done the research.
One thing that every market professional is worried about right now is just how dysfunctional valuations and returns seem to be in the modern era. Stocks seem to go up for no reason and returns have been disconnected from both fundamental and quantitative risk premia.
What I can say for sure is that we are in a period of intense change in the financial system. No one knows what the future of finance will look like, 10 years, 20, or 30 years out. Will crypto defi take over? Will traditional finance be disrupted? No one knows, but it is certainly an exciting time to work in the capital markets!
