only thing it would effect gameplay wise is the texture/model popping would be less, everything else will be the same. also too getting a 7200RPM drive isn't a bad call and I haven't really noticed heat generation being bigger (I have a 5400 and a 7200 and they stayed the same temps at full power when left alone for 30 minutes).
for the most part it is that severe, it's like hitting the vram limit where the performance just tanks and will keep tanking until settings are adjusted to keep it below that limit. check to see if you're using more than 3.5gb of Vram, use Afterburner or whatever you use to monitor you usage.
I had CTS once before, still kept doing my thing that got me CTS but since the I never got it again. right now I'm dealing with a shoulder injury (Dislocated my shoulder 5 times) so all is good though as I prefer CTS than my shoulder injury. .