Except approximately 2/3rds of BM’s damage leech-heals their pet rather than the hunter. 75th percentile damage output for BM on mythic Shad is 4810.31 DPS, of which approximately 1600 DPS actually leech-heals the hunter. That’s approximately 160 HPS from leech, and and only if none of it is overhealing (which is almost never the case).
Spirit Mend heals for a total of 258% AP over the duration. A mythic raider is likely to have approximately 1500 AP at the moment (based on a random sampling of upper-end logs, AP equals approximately 136% of Cobra Shot’s average damage outside of Bestial Wrath), which means Spirit Mend heals for 3,900 per cast (this also matches said logs, where they used spirit beasts, which was most). If used on cooldown, that’s 130 HPS. Slightly lower, but since it is intentionally cast, it’s much more likely to be useful healing (a quick spot-check of Shad’har, for example, showed the few hunters I sampled were at maximum HP perhaps a third of the time, including almost the entirety of phase 1. This isn’t unusual for raid boss fights), and it is freely targetable on raid members, and you also get a second defensive cooldown and a bit of bonus HP to boot.
Spirit Mend may have been nerfed, but unless you need a snare or mortal strike on a fight (and none currently do), it’s still the best option for BM for raiding. The healing is in the same ballpark as the maximum possible healing from leech, but is more versatile, more focused at the time you need it at, and comes with a second defensive CD.
From a talent perspective, Aspect of the Best boosts the leech from 10% to 15%, but it also boosts the healing of Spirit Mend by 30%, so it doesn’t tip the equation very much. From a gear perspective, we currently have late-expansion levels of mastery, which inflates pet damage. However, we also have insane amounts of crit from corruption stacking, which leads to an absolute glut of focus and a much more significant increase in Cobra Shot damage. Our damage in SL is actually even more biased towards the pet, further weakening the leech.
Basically, spirit beasts are weaker than they were, but they’re still the strongest option for raiding and probably for M+, unless you need the lust. For soloing, Ferocity pets were already substantially better, they are simply even more so now.