How many who have done the mod on their exhaust have removed the cats as well?
Contrary to what others have said, I found that removing the CATs worked wonders in fact. Just look at the restriction they present to flow - those are full fat 400-cell structures that to flow need massive volume (which is why effective, efficient stock CATs are huge in modern cars), but in the M3W they are stuffed into a tiny 2" header. I removed them first, before any other mod and immediately noticed the difference. Most M3W's fail the UK emissions test anyhoo even with the CATs (when tested as a Class-4 4-wheeler, as in theory they have to be, weighing over 450kg), so my car at least was better off without them. But yes, everyone's comments about Stage-1 minimum are entirely valid - this is not a performance mod, but an engine life expectancy mod in fact.
