layer1 (1:4) is 384kbit/s.#
This is the highest as far as I know.
Do you want higher? Why?
Does anyone offer software to compress SACD or Digital Audio CD
from 192Ksample/sec 24bit down maybe 10 times?
A high quality MP3HR (hi rate) that might just be better than 16bit CD.
mp3 (mpeg1 layer3) is limited to 320kbps, just for portability reason.
mp2 (mpeg1 layer2) - 384kbps.
mp1 (mpeg1 layer1) - 448kbps.
For 384kbps compression, you can use mp2 encoders, builded into video-related compression software like TMPGEN or ToolLAME. Process only audio stream, of course. You get output file with *.mp2 extension. Just rename it to *.mp3 - it playable on most software players (WinAmp for example) and recognizable with most of mp3-related software (for tagging or browsing or something else). Unfortunetely, onle few hardware players are capable to deal with mp2-stream, so using mp2 is limited to computer.
As for quality - 384 sounds virtually identical to 320 (with 20% apprx. rise in file size).
Remember, that gap between mp3/320 and uncompressed audio can be filled with lossless codecs like FLAC or APE.