The max power an engine can make period and the max power an engine can make while still meeting emmision requirements is another thing completely. Then there is the question of how much work said engine can handle at X power without the EGTs going through the roof and needing to defuel the engine. When towing, an engine that makes 350hp/700tq all the time can be better than an engine that makes 700hp/1100tq going downhill only. The 5.9l, 6.6l, 6.4l and etc can make 700hp too but its not good for much other than racing around for short periods of time.