Use the same method that x264 uses for P-skip detection.
This significantly improves quality (1-6%), but at a significant speed cost as well (5-20%).
It also may have a very positive visual effect in cases where the inaccurate skip detection resulted in slightly-off vectors in B-frames.
This could cause slight blurring or non-smooth motion in low-complexity frames at high quantizers.
Not all instances of this problem are solved: the only universal solution is non-locally-optimal mode decision, which x264 does not currently have.

subme >= 7 or <= 2 are unaffected.