Have you tried blocking the exhaust with your hand to see if there are any leaks, ? I find it easier and quicker to do it that way as it builds up its own pressure to test it and its quicker than using any smoke machine. ( has it got a drain hole in the back box. ?? )
I would be trying a fuel treatment /injector cleaner and also substituting the air flow meter as trying to prove that it is slightly out of tolerance is almost impossible.

It looks as though you have really done your homeork on this one but theres likely to be a simple solution.

Tony.