A change from the stock exhaust might move the powerband down the rpm scale a bit... but the peak values you're getting are higher than what I'd expect from a stock motor. I wouldn't complain.
Note that while the SAE specs have the HP peak at 5800, the more realistic DIN specs -- which take into account things like the engine having to spin a fan, alternator and water pump -- have it at 5500.
It is odd, though, that the torque and HP peaks are 200 rpm low from those DIN values, and also that the pull stops at 5800 instead of the 6000 you thought you ran to. Can their tach be off by 200?
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