Am I overclocking a PIC18F4525?
No, you can't enable the PLL with a 20 MHz crystal since 80 MHz clock rate is way out of spec.  The PIC may appear to work, but there is no guarantee what it might screw up at what time without warning.  Overclocking is a no-win game some clueless people engage in that think there is a free lunch to be had.  You said "product", so you really really don't want to go there.