No, it doesn't waste RAM. People think it does because they see Task Manager saying so much is being taken, and it leaves less RAM idle, but think about it. Idle RAM is wasted RAM at any given moment. Prefetching keeps commonly used things right in memory in case they are used, so the things you use are ready/done quicker. It puts your RAM to use. If you start running low, prefetching should drop said contents (probably the last/least used), so no, it shouldn't be "wasting" RAM. It makes better use of it than leaving it idle. I don't know the super details of it, but that's about it in a gist. You shouldn't disable it (unless maybe your physical total amount is 512MB or something crazy like that).
I never disabled prefetching, and yet still never seen it over ~1.25GB (maybe 1.5GB at most) when sitting idle or doing basic things, so I was honestly questioning how well it was even working? It doesn't seem overly aggressive. I assume Photoshop and some games bring it up over that obviously, but it goes back down after returning to the desktop. I'm wondering if some of the tweaking you did led to this. Maybe prefetching is working, but in a messed up way, and is prefetching things but not releasing it?
Can you check Task Manager when RAM usage is up and find out what is taking so much of it?