how do I determine compressibility gains from avisynth filters

[supernater]

What is the best way to test whether a particular set of avisynth filters improves compressibility?

My current approach is to keep my x264 settings constant but and do a single pass constant quality encode (using crf = 18) with one avisynth filter set and then do another encode using a different avisynth filter set. Then I compare the size of the compressed movies from each set of filters. If one movie is smaller than the other I conclude that the avisynth filter set that generated the smaller movie is the set that improves compressibility the most.

Is this the correct approach to compare the compressibility gains of avisynth filters?

[LoRd_MuldeR]

To check the "compressibility" I'd use CQP mode instead of CRF mode.

That's because the filters may also influence CRF's complexity measure. Remember: CRF raises quantizers in "complex" scenes, MB-Tree even raises quantizers in the "complex" parts of a frame.

With CQP you can be sure that there are no such side effects that may bias the result of your test. For the final encode I'd stick with CRF mode of course...