The most important thing about this is that it would never happen because of the political implications. If Texas were to become five states, it is very likely that the balance of power in the US Senate would be severely altered for a long time to come. It is likely that all of the five new states' US Senators would be from the Republican Party, thus giving the Republicans a huge advantage in trying to take the Senate from the Democrats. For that reason alone, it is extremely unlikely that this would ever be allowed to happen.
So, the main thing that would happen (that is of national relevance) if Texas became five states is that it would make it much easier for the Republicans to take the Senate and would also help (though to a lesser degree) with making it more likely the Republican presidential candiates would win most elections (at least 10 more Electoral College seats).