Giving minecraft more RAM will NOT increase FPS. In fact, the less memory you give it (as long as it's enough), the higher your FPS will be. Minecraft (or any application for that matter) will use as much memory as it needs, but will also keep track of all the memory it has allocated. This means that if you allocate more memory than it needs, it will have more to keep track of, which results in fewer FPS.
Used memory: 4% (505MB) of 7851 MB
Allocated memory: 100% (7851MB)
But how do I force minecraft to use all the ram I give it, resulting in more fps?
Note: I am not a troll and I have no idea if this is possible or not, any help is appreciated.
I made my own shader pack, by the way.