The overhead is, I guess, for the main SH2, since it get decompressed by the SH2, then it's no different from uncompressed audio once it's transfered to the audio RAM.
It could be that the SBL library isn't super efficient, but even for uncompressed audio it's still expensive to use PCM (even if it's "just" 5%.
And more importantly it hogs the B bus, which is the same bus used for the vdp1 and vdp2.
For the DMA, it's always SCU DMA on the b bus.
CD audio goes straight to the DAC, so it doesn't consume cpu power (the SH1 can't be used for anything else than loading). It's the easiest option.
Then sequence audio is great as well, you have the SCSP and the M6800 just for doing that, it has it's own RAM, it can playback PCM data (from its own memory), so it doesn't consume cpu power except for transfering the play commands. It's a bit tricky to use, but for fx sounds it's a no brainer.
PCM audio from CD is good to play voices or play music while loading data, but it should be avoided as much as possible.
PCM audio from memory is easy to use and convenient for that, but it really should be avoided.
I don't know if it's possible to keep data in the CD buffer, so that you could store 512 KB of PCM data in that RAM, but it would be a good option (if possible) to at least save some main RAM if you have to use PCM memory playback.
I would really like to use MIDI audio as it's the more flexible option and keep the CD available, but it's also the hardest to use.
EDIT : The program I mentionned which converts sequences to MIDI. Now we need to figure out how to do the other way around!
https://github.com/mistydemeo/seq2mid/find/master?q=