The OP was originally talking about an MP3 exhaust. The OEM one of which may be less restrictive than one from say an ET4.
So we cannot say for sure that all model's OEM is as good as every aftermarket ones. After market ones may or may not have CATs so maybe less restrictive but this only means at high rpm. But the may therefore also be less effective down low but with CVT we don't get "down low". So that depends on the riders style/terrain/locale.
For the 150's the new "Sprint" class are said to have less restricted exhaust than the preceding modes but this may also include other 3v engines like LX/LXV.
I know nothing about later model GTS exhausts. HPE and the like.