If you feel adventurous --hh corresponds to CONFIG_VERY_HIGH_FLAG, and --h to CONFIG_HIGH_FLAG. grepping those shows you where they're used which you can then investigate to see exactly what is enabled/disabled(the tool grep is a good first step to picking apart unfamiliar code):
grep -r CONFIG_VERY_HIGH ./
./src/common_utils.c: if (wpc->config.flags & (CONFIG_HIGH_FLAG | CONFIG_VERY_HIGH_FLAG)) {
./src/common_utils.c: if ((wpc->config.flags & CONFIG_VERY_HIGH_FLAG) ||
./src/pack.c: if (wpc->config.flags & CONFIG_VERY_HIGH_FLAG) {
./src/extra2.c: if (wpc->config.flags & (CONFIG_HIGH_FLAG | CONFIG_VERY_HIGH_FLAG))
./src/pack_utils.c: if (config->flags & CONFIG_VERY_HIGH_FLAG)
./src/extra1.c: if (wpc->config.flags & (CONFIG_HIGH_FLAG | CONFIG_VERY_HIGH_FLAG))
./include/wavpack.h:#define CONFIG_VERY_HIGH_FLAG 0x1000 // very high
./cli/wvtest.c: res = run_test_extra_modes (wpconfig_flags | CONFIG_VERY_HIGH_FLAG, test_flags, bits, num_chans, num_seconds);
./cli/wvtest.c: else if (wpconfig_flags & CONFIG_VERY_HIGH_FLAG)
./cli/wavpack.c: config.flags &= ~(CONFIG_HIGH_FLAG | CONFIG_VERY_HIGH_FLAG);
./cli/wavpack.c: config.flags &= ~(CONFIG_FAST_FLAG | CONFIG_HIGH_FLAG | CONFIG_VERY_HIGH_FLAG);
./cli/wavpack.c: config.flags |= CONFIG_VERY_HIGH_FLAG;
./cli/wavpack.c: else if (config->flags & CONFIG_VERY_HIGH_FLAG)
./audition/cool_wv4.c: config.flags |= (CONFIG_VERY_HIGH_FLAG | CONFIG_HIGH_FLAG);
grep -r CONFIG_HIGH ./
./src/common_utils.c: if (wpc->config.flags & (CONFIG_HIGH_FLAG | CONFIG_VERY_HIGH_FLAG)) {
./src/pack.c: else if (wpc->config.flags & CONFIG_HIGH_FLAG) {
./src/extra2.c: if (wpc->config.flags & (CONFIG_HIGH_FLAG | CONFIG_VERY_HIGH_FLAG))
./src/pack_dsd.c: if (wpc->config.flags & CONFIG_HIGH_FLAG) {
./src/pack_utils.c:// o CONFIG_HIGH_FLAG "high" compression mode
./src/pack_utils.c: config->flags &= (CONFIG_HIGH_FLAG | CONFIG_MD5_CHECKSUM | CONFIG_PAIR_UNDEF_CHANS);
./src/pack_utils.c: wpc->config.flags |= CONFIG_HIGH_FLAG;
./src/pack_utils.c: if (wpc->config.flags & CONFIG_HIGH_FLAG)
./src/pack_utils.c: int divisor = (wpc->config.flags & CONFIG_HIGH_FLAG) ? 2 : 4;
./src/unpack3_open.c: wpc->config.flags |= CONFIG_HIGH_FLAG;
./src/extra1.c: if (wpc->config.flags & (CONFIG_HIGH_FLAG | CONFIG_VERY_HIGH_FLAG))
./include/wavpack.h:#define CONFIG_HIGH_FLAG 0x800 // high quality mode
./cli/wvtest.c: res = run_test_extra_modes (wpconfig_flags | CONFIG_HIGH_FLAG, test_flags, bits, num_chans, num_seconds);
./cli/wvtest.c: else if (wpconfig_flags & CONFIG_HIGH_FLAG)
./cli/wavpack.c: config.flags &= ~(CONFIG_HIGH_FLAG | CONFIG_VERY_HIGH_FLAG);
./cli/wavpack.c: config.flags &= ~(CONFIG_FAST_FLAG | CONFIG_HIGH_FLAG | CONFIG_VERY_HIGH_FLAG);
./cli/wavpack.c: if (config.flags & CONFIG_HIGH_FLAG)
./cli/wavpack.c: config.flags |= CONFIG_HIGH_FLAG;
./cli/wavpack.c: else if (config->flags & CONFIG_HIGH_FLAG)
./audition/cool_wv4.c: config.flags |= (CONFIG_VERY_HIGH_FLAG | CONFIG_HIGH_FLAG);
./audition/cool_wv4.c: config.flags |= CONFIG_HIGH_FLAG;
flac -8 is far from the slowest encode setting you can use for flac. -8p seems to be the slowest in common use with non-negligible efficiency gains, but there are many settings an expert can tinker with and you can basically make the encoder as slow as you like.
Here's some encode/decode stats but it looks like they only go up to x4hh, just plonk x6hh somewhere to the right of that for an estimate: https://hydrogenaud.io/index.php/topic,122508.0.html