X flow done numerous x pipes samples with each being dyno tested on same car. They have refined the x pipe to increase performance.
My friend had a E92 M3 that had a van de linde full xhaust. Had a big whole in the torque curve at low rpm. We changed just x-pipe and immediately torque was back at low rpm.
in comparison with sherwin old m3 wichhad same stage 2 map. With both cars rolling from 2000rpm in 2nd his car opened gaps on my friends M3.
after doing xflow xhaust both cars were same.
Good luck with which ever option you go with.