Catergreat,
There are a lot of variables in the heat of time of a 250. They include the amount of food in the unit, the temperature of the food, the relative humidity and several others. So, it is hard to give an exact heat up time. If you want to check the unit to see if it is working properly, run it out empty. It should take right at 30 minutes to reach 200 degrees.
The problem with adding more heating elements or elements with higher wattage is that you either have to add a fan to move the air, or you will burn up the food on the bottom grill.
Hope that answers your questions, if not ask away...