I also tried this with 3 of my HDCDs, and found that for tracks encoded using "peak extension" you can gain considerable compression improvement by using very small block sizes. I found that --blocksize=4410 worked nicely, and on some tracks --blocksize=2205 was even better. And I'm talking like 5-10% better compression!
These are the results I get for the Joni Mitchell "Blue" HDCD, with Monkey's Audio 3.99-u4-b5, wavpack 4.41.0, flac 1.2.1 (all under linux):
16-bit WAVs (not HDCD decoded)
176.9 MiB mac -c5000 (Monkey's Audio insane)
185.4 MiB wavpack -hhx6
191.1 MiB flac --best
As expected, here Monkey's Audio compresses much better than WavPack, which itself is substantially better than FLAC.
24-bit WAVs (HDCD decoded), default blocksizes
338.9 MiB mac -c5000 (Monkey's Audio insane)
210.6 MiB wavpack -hhx6
198.8 MiB flac --best (default blocksize: 4096)
I thought there was something wrong with my setup, until I saw Walrusbonzo's results, confirming mine. I can't believe how bad Monkey's Audio fared.
24-bit WAVs (HDCD decoded), custom blocksizes
196.6 MiB wavpack -hhx6 --blocksize=4410
197.1 MiB wavpack -hhx6 --blocksize=2205
196.1 MiB mix of the above (smallest encodes from each)
197.1 MiB flac -l 12 -b 2048 -m -e -r 6 (equivalent to --best)
Now we're talking! Overall, wavpack with a blocksize of 4410 fares better than with a blocksize of 2205, although some tracks (4, 6, and 7) are indeed a bit smaller with the latter.
Note: Monkey's Audio doesn't provide the ability to change blocksizes.