@Acaila
I said to be careful about doing it, not to not do it at all
And I was thinking in terms of JPEG compression, I was unaware that MPEG handled quantization differently.
Can DCTed coefficients be larger than 256 (Or i just 128 if it rounds)? If it can, then even a QM w/ 256 coefficient isn't guaranteed to zero. In that case I still think that using the DCTfilter to zero coefficients is a better alternative than using a custom quant matrix. Otherwise, putting 256 should be sufficient.