No you were right, a 3/4 HP is more than enough pump. There is clearly something wrong with the plumbing. Cleaning the filters is a good first step because it is important to get a clean filter pressure reading to help with the troubleshooting. If the pressure still does not drop, then there must be something blocking the return line somewhere and unfortunately probably where the two side split.
BTW, what was the filter pressure with the old pump?