you psu will almost never cause you to limit fps. if its not poweful enough, a fuse will blow or it will turn off from excessive load. the psu cant tell the gpu when its drawing too much and then tell it to slow down. your psu is also a good model, there is no reason to change it if it works. the more likely culprit is your 8gb of ram. in modern titles, 8gb is starting to not be enough, especially with your respectable cpu and gpu choice. doubly if its single channel. you can try do some windows tweaks like gpd win scripts to reduce windows idle ram usage, close down other apps, or just a third party windows install like ghost spectre superlite to get your ram usage down and leave more for your game.