I think generally the park with EMH will be busier that day than other parks, but it will NOT be busier during the extra hours -- if you have park hoppers you can take advantage of the extra hours and then hop to a park that is recommended for the day -- or if the EMH are at night, you can go to a different park for the day and then hop over for the evening hours. If you don't have park hoppers and definitely want to be in the EMH park on a non-recommended day, you should just make sure you take full advantage of the lower crowds during the EMH and then have a good touring plan/use FP+ wisely for the rest of the day...lines are usually longest in the 11-4ish timeframe, so plan on trying to do lunch, take a break, watch some shows/parades, and/or use your FP+ selections etc. during that time frame!