ADC performance simulation: How to calculate SINAD from FFT?
If SINAD can be determined from only expected value and variance then it is possible to determine how SINAD transforms. Variance $\sigma^2$ is preserved whereas expected value $\mu$ grows as $ \sqrt{N} $ where N is the sampling set size. Noise + distorsion is assumed to have variance $\sigma^2$.   

Thus the SINAD value would then be determined to become
$$
{\mathrm {SINAD}}={\frac {P_{{\mathrm {signal}}}+P_{{\mathrm {noise}}}+P_{{\mathrm {distortion}}}}{P_{{\mathrm {noise}}}+P_{{\mathrm {distortion}}}}} = 
{\frac {N |\mu|^2+\sigma^2}{\sigma^2}} = N \cdot SNR + 1
$$

I can explain this in more detail if needed.