HD 7950 gets between 15-19 mh/s mining ethereum, is this the normal speed?

Hey everyone

I'm quite new to mining ethereum. I put a small miner together with 1 HD 5970 GPU under Ubuntu 14.04 with latest ubuntu repo's fglrx-updates and Accelerated AMD Parallel Processing. I'm using ethminer 1.2.9, geth 1.4.11 and all the other software required to do the job...
So far my system has been running fluent for a week but I was wondering I could increase the mh/s.

i start ethminer using the following command:

ethminer --farm-recheck 400 -G --opencl-device 0 -F [Dwarfpool URL, miner address.....] --cl-local-work 256 --cl-global-work 16384
I did not modify the GPU's setting of any sort, so no overclocking or similar stuff.

Three questions about my setup:

- The speed of the GPU is fluctuating between around 15 - 20 mh/s. Could there be a reason why it drops sometimes? Or should I not be worried?
- Could you yield better results with Windows using the Catalyst drivers?
- At the moment I'm mining with dwarfpool, like most people. What are your experiences with other pools, such as ethpool (1% fee instead of 2% at Dwarfpool?)

Thanks