For what it's worth, I ran across your post while looking into the same issue and almost the same hardware.

I have no performance issues, so until I can discover a definitive answer I have chosen to interpret it to mean that the high signal waits as a percentage means that the CPU load is extremely low, and therefore a high percentage - as opposed to a high absolute number like 50000 seconds - just means that when compared to the overall number of waits, that signal waits are the highest as a percentage of a very low value. 20% of 1,000,000 is a lot, but 20% of 100 is not.

Having said that, I continue to look for a better explanation.