One important thing I found out is that if you use OpenAL in any form, if "naturally" or through Alchemy, SBX is turned off and you don't get 7.1/5.1 downmix but a "standard" 2.0 headphone mix. Maybe it doesn't happen to you but it does in my case.
Makes sense. At least to my ears, they use HRTF in OpenAL (and obviously if you use Alchemy) when "hardware acceleration" (when using Creative's implementation instead of Generic Software) is enabled regardless of what settings you use. It also sound like elevation filter and macrofx (or what every they were called in X-Fi drivers) are enabled because Oblivion sound as buggy as with X-Fi when those two are enabled but you can hear things above and below you in Mirror's Edge.
The sound also changes if you use headphone mode or line out, I prefer lien out because the sound stage is much better. It's a shame that you can't use standard stereo mode when using headphone out and in that case you're stuck with headphone settings (which makes Mirror's Edge sound worse in my opinion).