It sounds like the things you have said check out fine. (Plugs, o2, exhaust tips, and smoke) pics of the plugs would be the easiest, if ur burning that much oil it should show up on the plugs, after that a compression test would b nice. Try going back to conventional and see what happens. You really should have vac applied to that rear valve cover, it's the fresh air intake side for the pcv system, and while I can't say what happens without it, I know it will prob work better to get the pcv system "flowing" the way it was designed.
AGENT J: full bolt ons...flashpro tuned by Dom.