510ish with wyvern exp sounds all right? A friend of mine was talking about hitting 650s, but not sure how true that was.
510ish sounds right. Your friend probably wasn't accounting for any TP bonus.
From my experience, it seems to be that the game doesn't check wyvern HP very often. It's very consistent that HP+ gear will take effect at the same time as my wyvern takes a swing. When he stops attacking to charge up a breath, the time to update his HP can vary wildly. I don't know any way around this, other than anticipating both wyvern and mob attack speeds.