Does the resampling and bit-depth reduction of the wav file actually help? My intuition is that a modern encoder like Opus will work better if it has more information.
I imagine it would be less of a size difference and more noticable at higher bitrates.
As for resampling, it depends on the specifics of the resampler used by a specific codec implementation. If a resampler is terrible, it could add artifacts like ringing and aliasing that would degrade the encoding quality.
A great souce for resampler comparisons: http://src.infinitewave.ca/