Except the 25nsec is not a set number, and neither is 5nsec/ft, both can be smaller or larger, not to mention there is hysteresis and settling time that pushes that transition values above/below 0V, which means that theoretical 12 and 15nsec could end up sitting on top of each other, which could happen anywhere from 1-2 meters system dependent. 1.5 meters may work, or it may be a worse case. There is simply not enough tolerance at 1.5m.
w.r.t fast edges, if the termination mismatch is not large, then fast edges will always be better. If the termination mismatch is large, you have the potential for again a worse case situation. It would be rarer, but still possible. The slow speed of TOSLINK couples with noise optical detectors and high gain. It is the worst of call conditions for jitter.
Of course, fortunately most system today are fairly jitter immune. Well $50 consumer DACs are relatively jitter immune. $5,000 audiophile DACs you are taking your chances.
w.r.t fast edges, if the termination mismatch is not large, then fast edges will always be better. If the termination mismatch is large, you have the potential for again a worse case situation. It would be rarer, but still possible. The slow speed of TOSLINK couples with noise optical detectors and high gain. It is the worst of call conditions for jitter.
Of course, fortunately most system today are fairly jitter immune. Well $50 consumer DACs are relatively jitter immune. $5,000 audiophile DACs you are taking your chances.