I have read through the manual on the Bravia, it says it takes 1080p on the component side YPbPr, but this model doesnt accept 1080p on the VGA(D-Sub) input. I cant use HDMI as I need to extend source which he used cat5 for. I am aware HDMI can be extended across fibre but there is no provision for it, it is for a friends house that already has the cable pulled.
I'm not sure about an answer to your question, but you might want to check out the AVS forums. They usually have dedicated topics for pretty much any TV that a person would buy.
The only thing I can think of is to try to turn off the EDID located somewhere in CCC. It may or may not help your situation. You could use Powerstrip to force the resolution, but it's not the easiest thing in the world to use. AVS can help with that though.
I will check out power strip and EDID in CCC, but surely It would be a standard setting on the Radeon 3870 using the RGB Dongle?? The card should not care about cable distances etc and yada.