Uhm, there is some misunderstanding about the "HD" world.
The gpu does almost nothing when playing blue-rays, if it did, well, would blue-rays players be sold for less than 60€/$?
Is more about the CPU rather than the GPU.
If you don't like the quality of HDMI/DVI-RGB, well, just go with the one you like. Or otherwise, tweak your video player settings.
There was too much talk about HDMI being superior quality over analog and stuff, but in reality, if you really want real HD, the one thing you want, is a real 2000 static contrast LCD.
I have had analog 2000static contrast monitors (which are more expensive than LCD these days, that have "50,000" and more dynamic contrast, of which you can see if you look closely, that those LCD's have 1000 static contrast written somewhere a bit hidden) and they where superior in every way. So HDMI is just a marketing tool if you ask me, or well, if you buy a 51 inch monitor it may do a big difference.
Some said, HDMI makes the image brighter and that's it, some say other things. But don't let it fool you, if with RGB looks better, then stay with that.
And HDMI for sound?, you wont notice any difference I tell you, just plug the old fashion way.
PPl need to realize, this words are just marketing tools, there is no relevant performance from the use of it (aside from that what we want to believe).