Our M3 is back from break-in and got her up in the air for a look on the underside. One of the popular modifications is what is called a Clutch Delay Valve, which limits the amount of fluid transfer/engagement of the clutch for smoother performance, but limiting performance for most.
Rather than go into detail, Zeckhausen has an excellent article here:
http://www.zeckhausen.com/cdv.htm
Location of the CDV on D/S:
Comparison of CDV and larger valve on bench - casting numbers are slightly different:
Comparison of CDV orifice and line fitting:
The CDV does not appear to have any restriction, there is no need for replacement/modification.