Well in the US, it is Federal crime to remove the cat on a vehicle operated on public roads. You might never be caught, or you might be. Fine and jail time possible.
There is no need for back pressure. Headers actually create negative pressure at the exhaust valve as the valve opens.
A striaght pipe on a 3.0 is VERY load. We fabrciated one for a friend's race car. WAY too load, even for the track (most track do have sound level limits).
'95 Alpine M3 LTW
'00 Dakar M Roadster
'02 Topaz M3
Red/White SRF #4 (Chassis 561)