are there any non-software performance related differences betweeen the SO and the HO ('03 & '04)? Do the aftermarket "boxes" remap the ECM timing and pressure parameters thus negating the factory SO vs. HO differences?

 

The only one I know about is the oil spray nozzels for cooling the pistons.

The Boss Hog
(Happy Monday !! . . . . . . )

 

The difference between the 03 SO and HO are the oil nozzles and programming.
the difference between 04's is programming and a catalytic converter.

 

There are alot of differences between the 555 and 600 engines. Most all of them are for emissions, but it goes way beyond just programming and adding a cat.