I've been thinking about changing developers for a while now. Those results you got a much closer to what I'm seeing when I'm developing for 1600 with Tri-X. I used the developing times on the Digital Truth page, 13th column down.

As for my Tri-X, the most comparable image I would have is this:
tumblr_mh3b8eCBMu1rsfqq7o1_r1_1280.jpg

You can see the grain is a lot tighter and less pronounced than the HP5+ which confuses me greatly! It's something I haven't been able to figure out, but boy do I like shooting my Tri-X at 1600!