I have the p900 and went through the same process. The short answer is that I have mostly given up on 5760 dpi and use 1440x1440 exclusively.
If you have glossy paper, you will see a very slight resolution increase with 5760 mostly only visible with a loupe.
Next, there can be a meaningful increase on maximum density at 5760, but it varies, and there's consequences.
At 5760, you'll need a different icc profile and linearization curve because it will print darker than at 1440. You might be able to get what you want just by spraying the print instead.
Next, at 5760 I almost always see bronzing on non-matte papers. To get rid of that, you have spray the print. If the paper is too glossy, the spray might not go on evenly enough for you and it will definitely change the sheen. For me this equation means that 5760 is not usable on glossy papers, which is exactly where you'd think it would be most beneficial.
So by default, I just stick with 1440 with black enhance on. If I know I'm going to spray the print anyway, then 5760 is an option.