New Portra 400
Okay, the main issue here is not developing (push processing or otherwise), it is exposure. Jonathan Canlas' shots on the new Portra400 pushed 3 stops are most likely not rated at 3200. When he meters he will overexpose the shadows 1 stop on top of metering with the incident meter pointed at the ground. More likely, he is actually exposing his mid-tones at ISO 400-800 (due to metering the shadows) then pushing. He is not rating properly. Canlas is an honest guy, and does know what he is talking about when it comes to film, but he is notorious for being cryptic and blaming confusion on idiocy. For instance, he says "And if anyone has worked on a movie set you know when they meter they have the bulb out and meter straight up and expose exactly what the meter says. I have found this is the way to meter for portra 400. No overexposing unless you like yellow photos." He probably has never shot motion picture film, and never metered for it, but makes a great assumption. The only reason a DP or Gaffer may meter that way is to get a highlight reading, and would detract the "bulb" to get readings from a specific light source. He told me once that if you point the incident meter at the ground you will cut off a stop of light, and if you detract the "bulb" you will lose one more stop. That is neither technical, nor accurate. Jon is a great photographer, and a decent guy, but pretends to know-it-all. IMHO... If you don't know his work, check it out!
We do shoot motion picture film quite regularly, and just compared it to the New Portra 400:
http://www.twinlenslife.com/2010/12/in-bleak-midwinter-new-kodak-portra400.html
The issue with grain is most definitely underexposure. Underexposure will always produce grain irregularity due to greater separation between tones. Overexposure produces less grain, when the the tones squish together as the dies settle in the high end of the exposure. B&W grain can be different when the film density is maxed out (more and more grain is stacked on top of each other), but still will have more visible grain in the underexposed negative. Take a look at the underexposed Portra in the aforementioned article. The grain is most definitely visible, even in the web sized images when the shadows are printed higher. That's why it is a good practice to put your mid-tones in a zone higher than they are intended to be printed, ie - metering shadows or rating mid-tones at ISO 200 for a 400 film. But with Kodak's Vision Technology, the new Portra along size the 500T both capture much more shadow detail, with less separation. And looking at both films overexposed 3 stops there is no "yellow" cast or added grain to the image. But I would venture to say, if you did rate it at ISO 3200 (placing your mid-tones into the lower zones of the film) then push 3 stops, the results would have more contrast and greater separation of tones, making the grain more visible. Without underrating the film you would just push your tones into the higher density end of the neg, thus no more grain when printed normal. I hope that makes sense.
I've gone on long enough now, time to let someone else speak. Happy exposing!