do a check on the 02 sensor. these are the symptoms i had before when i had no 02.. or also vacuum sensors(map) and lines.
disconnecting your map will make it sound like a 13B bridge port cause the idle to play alittle and give you a hard time accelerating.
02 sensor will just look like a chimney.. and eat up heaps of fuel.
the exhaust size is fine. on cold mornings it would only give out white smoke which is moistured water burning of the tip.
02 sensors can be had from around $50-70? at neapean spares st marys.. just a generic brand called acl or something does the trick. comes with the sensor and flange
also just try cleaning out the throttle bodies,carbies with a spray