mpsk demodulation using digital sample
Not mentioned but from what description you gave it sounds like you have no concern about the Transmitter and Receiver clocks being asynchronous (meaning drifting in phase relative to each other); I assume then that you have confidence in the phase lock of your recovered carrier. 

This would not be my preferred approach to do 8-PSK demodulation, but since you are on that path and had specific questions how you could pursue that route, let me offer the following suggestions. 
First, when you multiply the two signals (your carrier and the received signal) the result is the sum and difference of the frequencies (and phase) of the two signals. The sum is a 16 Hz signal that you would filter out, and the difference is the cosine of the phase since:

$$cos(\alpha)cos(\beta) = \frac{1}{2}cos(\alpha+\beta)+\frac{1}{2}cos(\alpha-\beta)$$

so
$cos(2\pi f_c t)cos(2\pi f_c t + \phi) = \frac{1}{2}cos(\phi)$ after low pass filtering.

Most significantly this alone has phase ambiguity since it only resolves 0° to 180° uniquely (for instance the output of 0 corresponds to both 90° and 270°).  This can be resolved by using two multipliers; one where you multiply by sine and the other by cosine to get full non-ambiguous 0° to 360° phase demodulation (One output is I and the other is Q and the result is mapping out a circle given by I + jQ. 

If you were only interested in phase resolution from 0° to 180°, then I would recommend a saturated multiplication (or specifically and equivalently an X-OR gate) as this would have a linear response of output voltage versus phase after low pass filtering. However in the proper IQ approach above, this would have the deleterious effect of turning the preferred circular response (with all phases then equidistant) into a square.

Probably already obvious but mentioning just in case that it is important to hard limit your signal prior to phase demodulation as the result is also directly sensitive to amplitude (a change in amplitude of the input will lead to a change in amplitude of the output). When you have sufficiently positive SNR, the hard limit operation on the phase modulated signal will actually give you further noise suppression as well (as it eliminates the AM components of the noise).

For other approaches in an all digital receiver, consider using a complex baseband signal and some of the phase detector approaches described in this link High modulation index PSK - carrier recovery.  You are already there with the approach of using two multipliers with a sine and cosine carrier with the addition of a Hilbert Transform to the input signal that is on the 8 Hz carrier.