Notice that a digital filter has two purposes :
1. Reconstruction (really only applicable to 44.1/48);
2. Bringing down high frequency "noise" (same signal but images beyond Nyquist (sampling rate / 2)).
What Arc prediction will do regarding Hires is #2 only. Oh, it will work on #1 as well, but this is quite moot because not needed unless we think that frequencies between 22.05 and 44.1 (for 88.2 sampling rate) are very audibe and need reconstruction too.
So what's left for the Custom Filter is bringing down the HF "noise" more but to a degree which is rather moot (because Arc Prediction at 705.6 (768) output already has it down sufficiently to be harmful (beyond 352.8 (384)).
So it's almost the other way around : When the Custom Filter is engaged by standard (for say 705.6 and anticipating 44.1/48 playback) it should not engage the filtering itself when Hires is in order and leave Arc Prediction as it is.
All again will be different when Arc Prediction is shut off in the first place (see grayed checkbox in the Filter Designer) and now *only* the Custom Filter is operative. But here too, you won't be able to make it yourself because it needs to show the results of it (though this I could graph) and now I also don't feel much like making such a filter for you guys because I just don't see the reason (for the better) for it.
Let me clarify what I understood because I think things are different at 352/384 than they are at 705/768 and I think that even hi res needs help. I understood that you might make a custom filter that behaves like AP when the source is hi.res. and like Custom (how it is now) when sourced with 44/48.
What I'm saying is that even with hiRes source material, at 352/384 my ear is much preferring Custom over AP. Hard to argue with the ear.
As an aside, I now hear the buzzing / zooming / mosquito-ing of the typical upsamplers that you've talked about. I also hear that AP never has it, nor do the interpolative filters in HQPlayer. With Custom, I can *sometimes* hear a little hint of the buzzing. I accept it as it comes with a bunch of other goodies. But usually when I hear it with Custom, it is in the recording itself; and I presume this is from the EQ processors that audio engineers use for vocals. No doubt they ring, some more than others. And I can check with AP for those recordings and sure enough the ringing is there for those recordings. It's not a clean ring when it is in the recording but the essence is surely there.
Anyway, I hear the trade off that you are making with these filters and can see that it is like walking a tight rope, picking the right balance between the two extremes.
I can say, it is amazing that Custom measures better than AP because AP sounds cleaner. But sound is sound.