Deterioration SNR for cubic interpolation?

How it is possible to estimate change ( deterioration) SNR on an output of
cubic interpolation at various number of samples on a symbol, for qpsk
signal?

How much will be worse snr (for cubic interpolation) in case of 4 samples
on a symbol in comparison of 10 samples on a symbol, for qpsk signal?

Whether there is any technique for the answer to a question?

here:
http://yehar.com/dsp/deip.pdf
And don't worry about the pink elephant...