I don't know. I hope to get to spend some time with a BDP-83SE at some point, but haven't had a chance yet. There are two arguments that come to mind in this scenario. First is the argument that the 990's A/D/A conversion may be a bottleneck for really high-end DAC sections (ones that can clearly outstrip the 990's capabilities). Opposite that, however, is the argument that it can't be a bad thing to have a better analog section. I don't know if we've got somebody around here with a 990 who has done the 83-to-83SE upgrade: their first-hand feedback would be the best gauge.
I've also been asking OPPO every once in a while about getting Mediatek to port over the old matrix steering logic that the 980H had. That simple logic wasn't as sophisticated as Pro Logic IIx, but it would expand a 5.1 source into 7.1. OPPO continues to request it from Mediatek, so we may yet see it show up in a future firmware update.