Are tax reliefs not fully calculated in stock prices?

After months of delays and postpones, tax cuts are getting closer to reality. They, to a large extent, were at the base of the growth of US indices, leading them to new historical records.
But JPM, the US investment bank, has good news for the investors - the growth is not fully reflected in stock levels.
In a letter to investors on Monday, bank analysts said tax cuts were reflected in just 50 percent of stock levels. This gives space for the index to rise to 2,800 points by the first months of next year, according to analysts.