I do have some experience with pushing film in FX-55, but not with HP5+ at 1600. As a matter of fact, HP5+ at 1600 is at the top of my to-do list.
Are you in a hurry? If not, I plan to start my testing in the course of this week and I should have some results the next weekend, of just after the weekend.
Two things to consider. I use the Gainer variant of FX-55 (liquid part B) and I have standardized on 25 deg Celsius.
No I'am not in a hurry, just retired

...
I used to push-develop Tri-X in X-Tol, but as I have a lot of basic chemicals still sitting, and almost perimated, in my darkroom, a left over of a long professional career, I want to use these, and, last but not least, Tri-X must be payed by it's wight in gold now.
An other reason is that I read that FX-55 should be good for scanning, so this might be an alternative for Pyrocat-HD, which was good for that purpose too, but isn't that health friendly.
And yes, I was planing to use the Patrick Gainer variant for part B, as I have a almost fresh 1l bottle of TEA standing there...
I would like to land on an 20°C process (I don't mind the time).
Ik ben benieuwd naar jou bevindingen, ondertussen doe ik ook enkele tests, we zien wel.
Tot wederhoren.