But NO engine is sold packaged with a tuned exhaust. All are sold with a cheap exhaust diverter or nothing for the exhaust at all. So which tuned system does one rate an engine with? As you noted, a decent exhaust system is a power multiplier, so those willing to spend many, many hundreds of dollars (likely more than the engine cost) could easily stack the power deck by finding and experimenting with the exhaust.

So what exhaust should be used for an accurate power comparison? Logically it would be none for a baseline comparison. Those wanting the most power are dependant on those that have already spent the time and money or need to be willing to toss the dice with their own time and money. Realistically they will need to buy a few different engines and do a lot of exhaust work to find "the one".