Can you really notice any improvements graphics wise?
Back compatibility isn't really about graphical enhancements...
The enhancements that do exist are limited to things like scaling and frame rate...and that goes for all the backwards compatible games. They play smoother and look like they have been polished...but "graphical improvements" would be a stretch...as that was never the point. It's no different than running something like a Gameboy emulator on a PC and saying "look how crisp everything looks when it isn't hindered by the limitations of a Gameboy".
So, yes...and no. Better hardware is always going to make an improvement...but the source material still hasn't changed...unless it has specifically been remastered, which Red Dead hasn't.
To catch up on this i haven't noticed anything but then again i'm playing RDR again after playing it originally on the Ps3.