Just to add another bit of experience. One of the bolts holding the EGR blanking plate on my manifold snapped last Christmas and exhaust gas was leaking into the engine compartment. I noticed a lack of power and lower top speed. So if there's not a good seal between manifold and block this will take your power down. The engine bay rapidly become coated in soot and I think it was running a bit rich because a trip that usually costs me £30 of fuel cost nearer £40. So if the manifold isn't perfectly gas tight it might affect the amount of smoke too. Maybe the ECU isn't seeing the revs or the boost it would 'expect' for a given fuel input so increases the fuelling.