With just one 4800 you are going to struggle for real time. If, based on GPU and resolution etc etc, you don’t get real time playback the latency between the UI and SDI out will be longer than normal. It is quite normal to have some latency as there is processing and buffering to ensure smooth playback but it will always be longer if your system cant play in real time.
If you are grading, use the SDI monitor, or in your case the HDMI.
We have a number of v9 beta colorists who have not upgraded to Lion and are still using Snow Leopard with no known issues. Its worth backup up your project database and trying v9. There are lots of speed improvements in v9.
In v9, try setting the video monitoring option to use the ‘Optimized display mode’ and the image scaling to Bilinear (be sure to set this one back to sharper filter before you render).
The scopes in v9 are also a lot better and use less CPU resources.
Peter