Overclocking anything is a balance between speed, temps, voltages, and noise.
Generally you have diminishing returns - as in it will take progressively more voltage to increase the speed of a CPU/GPU. The byproduct of that voltage, is simply heat, which must be removed in progressively more aggressive fashion to keep the CPU/GPU from overheating. This in turn requires more aggressive fan profiles which results in noise.
So - if you can live with the noise, and your GPU is cooperative, you could easily run the GPU at speeds and voltages resulting in stable temps of 85C. But - the associated noise would probably be annoying.
That's what the GPU manufacturers try to balance - they want to get the best speeds out of a given GPU at moderate temps with very low noise. This gives them a good reputation for fast, quiet cards which is a good thing. This also allows you to tune the balance more towards what YOU prefer by leaving some overclocking room in the design.
Overall, your setup is perfectly fine if that's what you're comfortable with. Fans aren't too loud, temps aren't too high, so you're good to go with the settings you have.