I think 2.5" would give the best increase in power for a stock 4lt. Any bigger and the gas will start to loose velocity.
Get a 'mandrel bent' one, as it will also help with the flow. Should cost somewhere between $200-300 fitted.
Ive heard that pacemaker are bringing out some new race series extractors, so it might be worth waiting for them. But if u dont want to spend that much, lukey, wildcat, hitech extractors will do the trick for around $300 fitted.
Depending on the condition of your cat, u might want to look into a new one. 2.5" hi-flow will set u back $200-300. But the standard falcon ones flow pretty good to start with.
Hope that helps
1st car EA Fairmont Ghia, extractors, exhaust
2nd EF GLi, 'custom' interior, extractors, exhuast
3rd EA Fairmont 5spd, low'd, extractors, exhuast, head
work, 3.45LSD, EL dash, interior+ El ECU
4th EB2 Fairmont, EL dash and interior, BBM, LSD etc etc
5th EB Fairmont Ghia 5L. All leather, lowered, exhaust etc