The consensus appears to be that there will be a short-term price pop, but the inclusion in the S&P also means there will be increased volatility. Increased volatility implies buyers will want a greater risk premium which might be a lower p/e or, with ACAS, as higher dividend rate.
But, whenever a study of this sort takes place they present the average or median result which overlooks the obvious point that 50% of the results were better than the median and 50% were worse.
Obviously ACAS will continue to trade on fundamentals once the dust settles, so we�ll be in the upper 50%