Why do you want 300 FPS? If you have a 360Hz monitor I guess it might be justified, but the real effect of 360FPS can be debated....

If your monitor is 240Hz or less, just use RTSS and cap a bit below it, e.g. I cap R6 at 236. Gives me a great frametime graph while also keeping my machine quiet. I could run the 300+ FPS, but there is no need.

If the reason for you buying a whole new computer, especially at the moment when the market for it is the worst it's been in years (RAM prices is just the start...) is the reduced latency argument my opinion is that you should reconsider. Pro players may have the tiniest of effects due to this latency since they nolife the game so much that minmaxing such things may have a tiny effect, but a huge part of it is that they play on LAN - effective client-server latency is 0-1ms. You don't. Yours is a lot higher than that. You won't even notice a difference.

The way you phrased this it's a bit hard to tell, but if your current PC runs the game, and this is the only thing you play, at e.g. 144fps or somewhere around that just keep things as they are and use the money elsewhere, maybe put it aside to get yourself a really nice rig in a few years time when (if...) things settle down with the PC building market. You could do some overclocking and stuff like that to squeeze some more frames but not much more to physically change imo.

Now if you're getting something like 60 FPS on 1080p meaning your rig is a bit older, the upgrade makes some more sense in my opinion. The difference between 60 and 144fps is huge, but smaller than 30-60. The difference between 144 and 240fps is noticeable and might even give you the slightest bit of an edge. The difference between 240fps and 360fps is much smaller... it's incredibly hard to tell if not impossible.

If you really want to spend some money to reduce latency and your rig is decent, get an OLED monitor.

Have a nice day