OC'ing a monitor doesn't degrade their life at all. Your not changing voltages, an OC either works or it doesnt. Sometimes if it 'works' it can also drop frames.
Its akin to OCing a CPU without touching voltage or using more power, there is no degradation.
Whats more likely to have happened is that ur running in a limited color mode like ycbcr 4:2:2 or 4:2:0 or RGB limited.
As for 60hz vs 75hz thats a frame time difference of 3,33ms
The difference between 144hz and 240hz is 2.78ms
Basically ur seeing more a difference between 60hz and 75hz than u do between 144hz and 240hz. And there are plenty of people who claim to tell the difference between 144hz and 240hz.
In the end however, ur talking about a 60hz monitor, chances are the pixel response time isnt fast enough for 60hz let alone 75hz, so ur not going to actually 'see' the benefits, rather ull 'feel' the smoothness, which can be achieved by running standard 60hz with 'fast sync' and allowing fps to hit 120hz.