Exhaust port porting?

The exhaust gases are supersonic & hot because of the cylinder pressure. They have an entirely different behavior than the intake charge. The goal is to kep them moving and hot. You are better served keeping the port area smaller and raising the roof to help it turn than anything else. That gives the least surface area for thermal trasfer and the port acts as a nozzle between the cylinder at the later part of the combustion cycle where the pressure within the cylinder is still fairly high and the cooler and larger volume header primary tube. That can help scavenging which is much more significant to complete cylinder filling than the pressure differential created by the piston moving down on the intake stroke. The step between the header flange and port does not hurt you in the least.