Bryan; Perhaps the following will help you out.

Yes the gravitational potential energy of the system at impact is equal to

U = - [G * Mp-e * Mh]/R = -5.95 X 10^25 MJ.

However, not all that potential energy will be converted to heat (something you keep telling me I don't believe,... but its actually you who assume all the potential energy is turned to heat, thus giving the largest possible increase in heat).

A lot of this potential energy will be used in reshaping the 2 planets into one.

For example, a cubic kilometer of mass on the surface of Heaven (radius 4800 km, say) needs to be "lifted" one and a half thousand kilometers to the surface of Earth (radius 6371 km), or more, if the cubic kilometer of mass started below Heaven's surface.

This takes a huge amount of energy.

So you get the idea.

Its not as simple as one may think.

It is this sort of thing that forced me to adopt the approach using gravitational binding energy (in the paper).

Earth formed from a collision
www.preearth.net

Plate-tectonics is wrong
www.preearth.net/plate.html