10.3 and 10.4 seem a little old...I'd personally try 10.7 and see what happens from there. 
SDK 2.1 is what I'm using with your same OS with quite a bit of success. I'm getting very similar speeds to you while using my computer.
C:\Phoenix\phoenix.exe -u http://REDACTED@deepbit.net:8332 -k poclbm VECTORS BFI_INT FASTLOOP AGGRESSION=7 DEVICE=1
Edit: Try turning the Worksize off...BFI_INT seems to somehow mitigate the need to process the work in smaller pieces. At least, it feels that way. No change in speed at all after the new miners were installed.
SDK 2.1 is what I'm using with your same OS with quite a bit of success. I'm getting very similar speeds to you while using my computer.C:\Phoenix\phoenix.exe -u http://REDACTED@deepbit.net:8332 -k poclbm VECTORS BFI_INT FASTLOOP AGGRESSION=7 DEVICE=1
Edit: Try turning the Worksize off...BFI_INT seems to somehow mitigate the need to process the work in smaller pieces. At least, it feels that way. No change in speed at all after the new miners were installed.
Thanks for the suggestion, is it possible to install 2.1 with the 11.4 drivers perhaps? I could try that and narrow down whether it's the SDK or the drivers causing the problem.
Just for reference, my current setup gets about 278 MHash/s with aggression=7.