HP5+ is a fine film for pushing. I routinely push this film to 800 and run it in either HC-110 or Microphen- it gives excellent shadow detail in both. I also have no problem pushing it to 1200, 1600, or even higher- though you do generally lose some shadow detail the higher you go. For pushing almost any film beyond a stop and a half or so, I really only use Microphen (always full strength). It's one of the best formulas for push processing just about any film. I believe it was formulated specifically for use with HP5, but it certainly works very well with a great many films.
I use it slightly warm (70F, instead of 68F/20C), and as I said, always full strength. For HP5 at 1600, start with 11 minutes. Go easy with your agitation; remember the old basic rule of thumb- time controls density; agitation controls contrast. I agitate for the first 30 seconds, then two inversions per minute. Enjoy.