shroud tension for oversize wire

[Brion Toss]

Hi,
Too many anomalies here. Oversized wire should be tuned less, relative to break strength, not more. Remember, tuning is about dealing with elasticity. Also, note that there is no guarantee that the original rig was the correct size. What is the vessel's maximum righting moment? Next, "sagging in waves" is a less-than-precise description of the behavior of the wires. Finally, there are other reasons that you might be seeing slack rigging, even with high tune. None of those reasons are pleasant. I'd look first at the mast step/compression post/deck components.
Fair leads,
Brion Toss