I'd like to compare them in encoding speed
and transcoding efficiency
As I wrote in my previous post, there are still rare samples that are quite untransparent at 320 kbps and even higher, with all these encoders. It is just that they are rare, you must search for a specific type of signal..
Why a listening test at high bitrates (192kbps) wouldn't work:1) Most samples would have already reached transparency at that bitrate. And choosing only problem samples would make the test less significant since you wouldn't be testing a broad range of musical styles.2) Only a handful of golden ears would be able to reliably ABX the samples, and even after ABXing they would hardly give scores lower than 4.53) Since all scores would be around 4.5, the error margins would be big enough to make all codecs tied. To avoid that, you would need to have hundreds of participants, in an attempt to bring the error margins down.4) You would have a hard time finding hundreds of golden ears, and even more, hundreds of golden ears willing to participate, because the test would be very fatiguing and frustrating, due to the difficulty of ABXing.5) At the end, even if you managed to bring the error margins down, the codecs would be ranked so close that you wouldn't be able to produce a decent conclusion. All codecs would seem (or be) tied to each other.
And I thought the definition of transparency means "MOST people cannot tell the difference if it's the original or not".