Have you considered what happens to variable difficulty when multiple miners are using the same IP and the same payment address. For example one public IP is used for three miners behind a router and firewall but they have varying hash rates (180, 90, 70000) requiring difficulty 1, 1, and 64, respectively? This appears to be a problem even when using the new addresses for getwork for the first two and stratum for the third - where the slow miners usually end up getting difficulty set far too high and the fast miner occasionally has difficulty set too low. The same appeared to happen more than one miner used the new stratum address.
If you're mining on the same username, then no difficulty set by the pool is "too high" for sure. It can be too low for the sum of the connection through IF a) miners come from different IPs, or b) miners are using different protocols. If you have all of your miners on the same IP and username and protocol, the pool should properly assign difficulty.