98 GST blows white smoke at idle???

[98FWDSM]

ok, about 3 weeks ago i had a 3" RSR catback system put on with the stock cat and downpipe....while the catback was off the stock cat was beaten out and is now a straight thru pipe...it is still the stock cat put you can see straight through it...and then everything was welded together...now since then when you run the car hard it stinks...kinda like a old dump truck would, not sure if this is because of the cat being gone or what...and at idle it blows white smoke like it would in 20 degree weather upon a cold start up. Also, the check engine light comes on about 4 times a week since the exhaust install and says "catalyst efficiency below threshold" and i just keep erasing it and it keeps coming back on..not immediately but within a couple of days....and i just checked my oil and it was WAY LOW!!!....i havent smelt oil burning or anything so i dont know what is up with that?? so...is my O2 sensor gone bad or is it because i have no cat converter or what????

Thanks

 

[HondaStalker]

Ive heard that when you don't have a cat it will blow some smoke. My cousins smokes at idle on a fresh rebuild, and also blows out darker when he hammers on it. Who knows.