Having actually played and beaten RDR on PS3. I can say that some of the game would need to be changed in order for it to work in a PC setting.
The main issue off the top of my head, is how its controller based auto aim system is used during horse/coach rides. How it works is the game literally and fully auto aims you to a target just by using left trigger. It actually cycles targets with each left trigger press.
So riding along on a coach/horse its literally this: *Under attack* Left trigger-right trigger to shoot. Left-right, left-right, left-right. Thats an actual shoot out.
I'm a fan of mouse based aiming too, fully. But this game hand holds you. Maybe thats why they don't release it on PC. They don't want to have to re-do game mechanics.
- That's a very nice everything you have there... ssssssssSSSSSSSSSSS -