This can be seen 2 ways.
1. I would say a more "working man" test would be to shoot "heavy shielded boar," {or sow for that matter, nuther story}
Start with a 450g arrows at various speeds,
Try some 500g arrows at various speeds,
Try some 550's, then 600's, then 650"s, etc... etc... and determine at what point are you "RELIABLY" getting through the shield and at least to the off side rib, every time.
Once you achieve that you have just reached the point of diminishing returns of arrow performance. About 58# an 580g should get you there if tuned right.
2. Once you got an arrow speed that you can use out to your desired effective kill range, you have just reached the point of diminishing returns. For me it is about 7.5gpp.with my slower bows.