Ok did some research and here are my findings on this format.
The original wav file was 3:29 minutes in length and is stereo at 44.1Khz. Its size is 36,780,620 bytes before anything is done.
Conversion to Flac lvl 8 the file size is 29,087,203 bytes.
Conversion to APE Insane lvl the file size is 28,446,356 bytes.
Conversion to Mp3HD (Mp3 encoded at 320K CBR) the file size is
Flac compression 20.92%
APE compression 22.66%
Mp3HD compression 12.61%
Foobar 2000 was used for the Flac conversion using the Flac command line encoder. APE conversion was done through the Monkey Audio GUI. Mp3HD conversion was done through All4mp3 tools on site.
The Flac and Mp3HD encoded about the same speed. The APE file took considerably longer but had the better compression of the 3.
I tested the Mp3HD file in Winamp with the plugin provided on All4mp3. The plugin has a configurable menu that can be popped up when a Mp3HD track is played to ask the user which portion of the file to play, lossless or lossy. So after confirming that, I decoded the Mp3HD file back to wav and loaded it and the original wav up in Foobar2000 and did a binary comparison. The files were exactly the same so the Mp3HD format did what it said it would do.
My final thoughts, I think it is really pointless. Mp3 is for portability and you pretty much blow that out of the ball park having a lossless portion added to file size. Lossless compression is mainly for archival purposes or to be listened to at home. I did encode the wav file to a mp3 320 CBR with Lame and the file was 8MB in size. So it does have an advantage of being smaller if you took into consideration mp3+lossless from each format. However, like I said before, it kind of defeats the purpose what mp3 is for so why do it in the first place? Better to keep your lossless copy at home and use your mp3 copy for whatever on the go.