Nice! This experiment has given me newfound respect for Portra. But if any old color film can be pushed, then there's no real reason to go with Portra just to avoid push-processing...
The reason to avoid push-processing is higher quality negs
If not, keep on pushing
I may end up giving this a shot at 1600 through the pentax. I'm curious to see how scans will look with the Nikon. Also I have been told to shoot portra 400 at 640 then push process to 800. Supposed to give punchier colors and a snappier contrast.