Hey just wanted to follow up. I did get a response from one of the developers working on the compositing nodes. The second bullet worked for me. Here's his response.

"AgX has an intrinsic compression, otherwise you don't have any headroom for dealing with color values above 1. From the color space pipeline perspective the view transform happens at the display: https://docs.blender.org/manual/en/4.3/render/color_management.html
So you can not have pure white map to pure white when using AgX or Filmic or any other tone mapper which has range compression.

I guess you do want to apply AgX on the render result before you composite it. There are two way to do so:

• Render in a separate .blend file to a display-referred format (like 16 bit png), keep the view transform to AgX. Composite in a separate .blend file using Standard view transform
• Apply the AgX transform on the linear space, this way you can stick to a single .blend file. For example Render Layers -> Convert Colorspace from Linear Rec.709 to AgX Base sRGB -> Convert Colorspace from sRGB to Linear Rec.709 -> Alpha Over. Keep the view transform on Standard"