I think you'll need to read the timer sections of the chip's datasheet in detail for doing frequency modulation of a PWM pin reliably - the problem is when you shorten the cycle time there's a risk the counter becomes larger than the value of TOP and fails to wrap round until the timer itself wraps.
At 80kHz for a 16MHz processor you are talking about 200 system clocks per PWM cycle. But varying the pulse widths is going to vary that - might you need more than 256 cycles? That means using a 16 bit timer (timer1). Then the wrapping problem is a serious issue as a 16 bit timer wraps every 65536 clocks if you manage to skip past TOP by accident.
It would be much much easier to vary the duty cycle without changing the frequency.