Yes, perception is very important which is why we make listening tests. You need to eliminate (sub)conscious bias to get valid results. Any audio 'engineer' (sorry for the quotes, but engineer requires an academic degree over here) should know that.
"24 bit to MP3 involves two conversions... 24 bit to 16 and uncompressed to compressed." - No, it depends on what bit resolution the encoder can handle. LAME 3.99 can encode an MP3 directly from a 24bit file (the LAME encoder is one of the most, if not the most popular encoder in the mastering community, likely what most of the MP3s out there have been encoded with).
Sure, some encoders indeed can handle above 16 bits, but the resulting MP3 isn't accurate anywhere close down to 24 bits. The only difference between a dithered 16-bit and 24-bit file encoded as MP3 is noise. Low level noise.
Perhaps, the idea of keeping things CBR is (from other engineers who share this POV) to keep the encoder consistent; VBR will reduce the size of the file even more and when creating MP3s, my idea is to keep as much of the data as possible (a bit counterintuitive, I suppose). Pondering whether VBR sounds better than CBR is right up there with saying one fake Sugar brand is better than the next; they're still NOT real sugar ;-)
I don't understand 'keep the encoder consistent' but yeah your idea to keep as much data as possible is right. Since 320 CBR is the highest possible bitrate (higher than V0) you get the highest possible quality but also much bigger files, and: "but to date, no one has produced ABX test results demonstrating that perceived quality is ever better than the highest VBR profiles" (HA wiki).
Trust me, the only ones obsessing over things like ABX testing aren't typically the people responsible for delivering this content to the masses (tends to be really analytical audiophiles that love to argue more than just chilling out and listening to the tunes!)
Sorry but I don't.
"MP3 320 CBR doesn't filter out stuff above 16 kHz. As someone who masters audio you should know that." - I mentioned the LAME encoder, other encoders might not do it, but I mentioned that LAME is one of *the* most popular encoders, so likely some commercial tracks that people download have everything past 16kHz filtered out. Versions prior to 3.99 didn't, but I see a lot of MP3 files with missing frequencies past 16kHz; it's not that uncommon.
LAME also doesn't filter above 16 kHz with the highest settings. Not even with 192 CBR. If you get a LAME-encoded CBR file with everything above 16 kHz filtered out you got most likely a 128 kbit/s file - something I wouldn't even download for free.
On paper, this is the truth. In the real world, I'll tell you what I did one time. I grabbed my wife's favorite CD and made a 320kbps MP3 version of it and I popped it in the car stereo on our way to run errands. After about 10 minutes, she said "Something's wrong with this CD, it sounds like it's going bad or something." My wife completely hates anything to do with analyzing audio and is simply someone who loves music without having any affection to anything technical (my favorite kind of person).
Yeah right, you start believing things for good reasons. Theories are not based on hot air but for example on listening tests.