Don't think the smoke level will change, as that's unburnt fuel. That will change if you get a programmer or chip for it.
Less restriction for the turbo (backpressure), so it will spool up quicker with possibly more max boost pressure.
Not sure of all the ramifications on the 6-oh and what if any issues will result from this affecting the ecu and emission controls.
Also, emissions are steadily getting stricter on diesels in many states, and it may not pass vehicle inspection w/o it.
I'm a 7.2 guy, and yes it helped on mine (downpipe, straight thru muffler and 4" pipe).