How's performance improvement with crossfire with 12.1 over 11.11b or c?
It should perform worse than 11.11b or c preview drivers.
Remember there are two tracks for AMD drivers, the release (stable) track and the performance/preview track.
The stable track release in time for their monthly numbers, so november's stable release is 11.11. December's is 11.12. These drivers are tested thoroughly and only include features that have also been tested for a while. The preview track however are advanced viewings of features that might one day make it into the stable track. So 11.11b/c etc. contain new features that haven't been tested/polished enough to make it to the stable driver - but we can expect them to be included a few months down the line.
(if you want further distinction, performance drivers are ones with specific quick hotfixes, such as 11.11b/c, while preview drivers are advance viewings of a future possible stable track release).
This is a little confusing, but that's why they are hidden away so that only those people who are interested and can understand these things (and understand that they are not final/polished products) should be using them. If you are unsure, stick to the stable release set which is the one Steam will find for you.