Changing your miner's difficulty (up) is mostly to reduce your internet connection bandwidth used and the server load, but it will also affect your CPU load (just few % but still) -  adds some lag (reduced speed) to your hashing
Just like for the OC clocks for your GPU and mem, you should find the balance for your case ... between the load (connection and CPU) and the variance in your reward in short and long rounds.
For each pool and miner's hashrate it is different, but for slush i would go for 2-5 sec per share, which is exactly the suggested difficulty from the account page. If you are on bandwidth limited/expensive internet connection and may accept bigger variance in your reward - go for 10-15 sec per share

Are namecoin mining just dead on this pool? If so does anyone know of a pool with mergemining

Thanks again for sharing your extensive knowledge on the subject.

-G

Was all your rewards confirmed?

Have your recently changed your email address or payout address?

If you change your email address, I think he holds all payouts for 24 hours.  And if you change your payout address and don't confirm by email, he may hold payouts.  Not positive on the second one, he may either hold payouts, or just send 'em to the old address.  But it doesn't look like that happened...

I seem to be in the same boat. I decided to temporarily suspend mining and zero my account at Slush, so did the following steps:

- increased the payout threshold to a value higher than total reward (i.e. confirmed + unconfirmed)
- stopped all miners
- waited till all reward bacame confirmed
- waited some more
- decreased the payout threshold to a value below the current total (==confirmed) reward.

The automatic payment was executed some time later, but I do not see this transaction in my bitcoin-qt.

I will try to rescan the transaction chain before bothering Slush with it.

Can't remember for sure, but I think the payments are sent without a transaction fee -- if true, it can take some time for it to be picked up in a block.  I've seen 24+ hour transactions on Blockchain.

Difficulty is up to 50mil now

True haha