So I have an x-pipe with HFC and was thinking after I bought it that maybe test pipes would have been the better choice. I was happy though and never thought about it again. Well everyone and every company has tlaked about the "primary" cats (cats closest to headers) as the most restrictive however take a look at this official bmw m3 education manual and you will see that the "primary" cats are actually the non-obstructing more flowing metallic ones and the "secondaries" are the main cats.
Makes me wonder how much more people are getting with a true xpipe vs the test pipes as TP are essentially deleting the least restrictive cat and leaving the most restrictive.
See page 62
http://www.e90fanatics.com/pdf/BMW-M...nformation.pdf