This is probably far from the most efficient use of ROI, but I have Inner set to +25% and Outer set to -75%.
So it deprioritizes the decoration around the edge and prioritises the entire game capture, it might just be placebo, but it looks better than not using ROI and apart from some bitrate high and low spikes every now and again it seems to stay inside the 6000kbps(10200kbps aggregate for TEB).
Does ROI use the bitrate set in the Output Settings as the bitrate target both with and without TEB enabled?