I know there has been some discussion on this topic and I have also seen some successful examples of Provia 400X pushed to ISO 1600.
I have just got one roll of 400X I have exposed with a Mamiya 6 under different conditions. Most of the photos look underexposed. Even worse - there seem to be no true black in the slides. The slides also look rather flat. I will examine them into more details with a loupe on light table once I get home. But the first impression is not that great.
I will scan some of these, but probably will not manage before Friday (I leave for vacation and would like to used pushed 400X there ...)
So - I am wondering what is your experience. Do you expose the 400X and ISO 1600 and have it developed +2N or you expose it at somewhat lower ISO while keeping the development at +2N ?
How do you proceed when you want to push only to ISO 800?
Caveat I: The Mamiya 6 is rather new to me. But being aware of possible underexposure resulting of how the metering behaves I did for most occasions two exposures (one "direct" metering and one avoiding highlights in the viewfinder that did not belong to image like lamps or bright sky). At the same time I have exposed one Provia 100F and one Velvia 100 films with the same metering technique in in most cases only the "second" exposure is correct.
Caveat II: the lab I am using does always a great job (they do only slides) so I do not suspect any problems there.