It's my understanding that there are some technical limitations of the OSRS engine, but I'd really love to see this feature added. I'd like to see more than just the increase in FPS though.
It'd be amazing to see some of the things featured in the RuneLite 3rd party client that are powered by GPU rendering, like increased draw distance and skybox changes. It makes the game a lot prettier in my opinion, even though the only real change is being able to see further - and the horizon not being a black abyss.