Primary cats are most restrictive, confirmed by reputable tuners. See below
http://www.dinancars.com/university/detail.aspx?id=15
"Dinan will be making a middle racing exhaust system for the M3 similar to the one we manufacture for the M5 and M6. This racing exhaust will remove the second stage or rear catalyst"........ "This is an Ideal system for a showroom stock race car T-1 T-2 where the car must pass a smog test at the end of a race and cannot have a check engine light on. The gain about half of the front cat or 10 hp but there is no hassle and it is a lot less money. On Dinan’s middle exhaust system we also include 3 different noise level resonators so you can get the sound you are looking for from your car."