How big is your dump pipe? Sounds like the lack of back pressure that your silencer was providing before has caused some nasty boost creep, either that or your wastegate actuator/flapper has some issues, but its seems like a coincidence if there was nothing wrong with it before you took your silence out??
The big puff of smoke would have been caused by you hitting boost cut, I wouldnt worry about it too much if your car is still going! Idid it today when i missed 2nd gear, i cant drive with jandals (thongs) on! :lol:
What I would be worried about is the oil leak, perhaps the oil feed or drain has a leak?