PSoC has internal regulators which provides a regulated core voltage 1.1 V or 0.9 V. All the digital peripherals inside PSoC use this regulated voltage. So there will not be any significant change in performance at at voltages 2.8 to 3.3 volts.
Analog peripherals also work flawlessly at 2.8 volts VDDA. But we recommended to use higher voltages if possible. Because for example in IDAC, more the drop across the IDAC, more the accuracy of IDAC.