The + option (pseudoshare difficulty) is indeed disabled for the Bitcoin P2Pool due to me being paranoid, though that shouldn't be a large issue.
The other option (/) doesn't seem to be currently implemented anywhere (I PMd forrestv to find out about this)
The / option (share difficulty) is still there, though:
In work.py:
desired_share_target = 2**256 - 1
if '/' in user:
user, min_diff_str = user.rsplit('/', 1)
try:
desired_share_target = bitcoin_data.difficulty_to_target(float(min_diff_str))
except:
pass
...
desired_timestamp=int(time.time() - self.current_work.value['clock_offset']),
desired_target=desired_share_target,
ref_merkle_link=dict(branch=[], index=0),
I urge larger miners to use /3000 or higher in order to keep the variance down for smaller P2Pool miners. This will increase the difficulty of your shares to 3000, but each share is worth proportionally more. Simply append /3000 to the end of your miners' usernames. This will keep share difficulty (and consequentially, variance) low for small miners.
Also, don't worry, because no value is too high for either pseudoshare (+) or share (/) difficulty. P2Pool will ignore values of + that would cause it to drop shares otherwise, and the / value is capped at 10x the current minimum(default) share difficulty.