I'm a bit out of the loop, but, did something drastic change in 2.3.2 ?
I have rigs reporting as high as 170% Efficiency right now, with 125-150% being the average range for the others.Win7 x64, CGMiner 2.3.2
5 rigs each with either SDK 2.1 or 2.4 and CAT 11.12 on all.
Mixed 5xxx/6xxx series card machines, exclusive 5xxx series card machines and exclusive 6xxx series card machines.
The performance is not specific to any card/config. The ONLY thing they all have in common is the 11.12 driver and CGminer version 2.3.2.
WTF ?I double checked and didn't rely on the Efficiency% readout only. The ratio of GetWork's to Accepted's (even with Rejected included/excluded) does not lie.
Colour me impressed

Efficiency is the number of shares you find per work requests from the pool.
However, with RollNTime, a miner can generate multiple work from the one work request.
So on
my cgminer at the moment that shows Efficiency of 271% (which isn't actually very high) it means for each work request I ask from the pool, I generate on average 2.71 work requests (and on average reply with 2.71 shares)
The time it takes to mine those 2.71 shares is on average 2.71 times as long as mining 1 share.
A higher efficiency means that the pool had to provide you with less work and thus less communication and thus also less communication for your miner to the pool when trying to get work ...
Overall it's a big advantage for the pool and usually a small advantage for each miner
(but it doesn't mean you are getting more shares)