LED degradation should very rarely be a factor as long as the device is run under proper conditions. Temperature does not affect performance very much, because the LED and sensor have opposite temperature gradients, as one gets weaker, the other gets stronger. However, very high temperatures could negatively impact LED lifetime. A higher input current will increase how much light the LED outputs, which will increase the output current of the sensor. It can be advantageous to do this in some situations, but the contrast ratios should be re-verified. Also, a higher current can reduce the lifetime of the LED.