I am weary about the statement "the largest piping on the market" for the m3. This is a car that has PLENTY of pipe diameter and a perfect diameter to keep exhaust velocity fast enough to scavenge which any high revving NA engine really requires, or atleast is needed to perform the best.
Especially putting larger piping at the rear piping and keeping the same stock piping in the rest of the exhaust, you get a sudden increase diamter and subsequent big drop in velocity an thus it does not "suck" the exhaust through the system nearly as well.
If you change all of the piping from the headers back to increase diameter, than you great reduce the scavenging of exhaust out of the headers and thus the cylinders which impede intake flow due to lack of vaccuum.
Unless a car is FI, in which obviously vaccuum and scavenging is pretty irrelevent, keeping the OEM size piping is always ideal. IF you have FI and do big power mods than it is fine and desired to increase piping quite a bit as velocity of flow matters much less.