I can actually address your questions directly, because I have pushed HP5 to 1600 with both developers.
Here is a shot (these are scanned 8x10 prints) with HP5 pushed to 1600, as a matter of fact,
in Microphen in 1974. This is my violin teacher at UBC, John Loban, and the camera used was a Nikkormat Fn, with a 50mm f1.4 Nikkor lens, studio lighting (there was a bit of natural light coming in out of the window) at f8. I don't remember the shutter speed but I would guess at about 1/250th or 1/500th because he was playing the Ysaye "Ballade" for me, and that bow was moving fast. I remember the aperture because I needed some depth of field because he was moving a bit as he was playing, and I wanted his plant in the background to be somewhat recognizable.
The next shot is my former piano teacher become accompanist, Robert Rogers, as we were rehearsing the Beethoven "Kreutzer" Sonata for an Art Gallery recital in Vancouver. This was pushed to
6400 (!) in ID11, and taken in 1986, Nikon F3, Nikkor 85mm f1.4 lens, 1/15th at f2.8.
You probably can't tell much from the pictures except you will lose your highlights (they'll be blown even at 1200 or so), and that the grain with Microphen is a bit mushier (that's why I switched to D76/ID11 for pushing) - better clarity in the grain. Also the tonal graduation will be nicer with ID11/D76 - it doesn't seem to build up the contrast in the midrange as fast.
I'd personally recommend ID11D76 for the job - it's usually the pushing developer of choice. Also, I'd use another film besides HP5 - it balks at a lot of pushing, although you can get an image even at 6400, as you can see. For pushing, I actually used Plus-X in Ethol Blue, and preferred the midtones to HP5 in ID11D76. But like huffy above, I don't mind HP5 pushed in ID11/D76 at all - a nice flat negative and it'll look fine.
I know that the current Tri-X pushes more readily than HP5, although I prefer the latter film for pictorial use personally.
I'd point out that the apparently sharper grain in the first picture is more due to a bit of unsharp mask applied to the scanned enlargement than the actual print itself, where the grain is definitely mushier.