Probably a number of things, exhaust, and cat can make quite a difference as you said camshaft, compression ratio, ECU tune, ignition and fuel maps, and how much the EGR comes on, also the rev limiter. I read somewhere there are several versions of the head also. Also, there are different version of the intake manifold. oh and thick and thin head gaskets.
have a look here
https://club4ag.com/forums/viewtopic.php?t=22458
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