I just started mining this on the official US pool and I see it implements the terminally broken vardiff system so common to Cryptonote pools.
I'm doing about 720 H/s on the 5555 port and diff is already at 129922 after a mere 10 minutes. I understand that higher difficulty shares are worth more, but that is only true if they get solved/submitted in a timely fashion.
Usually one specifies a fixed diff on similar pools by inserting either a "+" or a "." in between and . I read the first page and the last 3 pages of post but didn't see this mentioned for ird.cashpool.us (I did see that a pool in France offers fixed diff, but ping time isn't too good for me here in the US).