Although there is no actual parsed data posted anywhere that I know of, at least one person with Spharai (Appie on BG) has indicated from his testing that the cap appears to be 80% (there might actually be parsed results with some of his posts, but my memory is pretty fuzzy on it). I know someone in game who has Spharai as well, and he has indicated that the counter rate doesn't go nearly as high as he'd hoped. Barring counterevidence, I'm willing to accept 80% as the cap.
It does make sense, in a way: your maximum evasion rate is 80% (ie: the mob's minimum accuracy is 20%). That counter would be the same is quite believable, even if we'd hope for better.