Testing in that way will only tell you if the two devices work with the given cable. It won't tell you what sort of margins you have. Not all devices are made e

Testing in that way will only tell you if the two devices work with the given cable. It won't tell you what sort of margins you have. Not all devices are made equal. A device isn't even necessarily better if it works with one cable when another doesn't, it might just happen to have a slightly lower impedance on its internal traces that happens to better match an out of spec cable, or any other one of a number of parameters.