So, because I'm bored... a three-turn XS3 damage average using the numbers in the topic, but where Jin uses Heaven Tracker & KOS / MOMO use Blood Dancer turn 1 is 2425. 2425*3*2.5 = 18188 for 1x PCHP for an endgame boss by this view (so yeah, Kevin clears 2 PCHP... unless you hold Blood Dancer & stuff like Overtaker more heavily against him due to the fight taking longer than 3 turns.). This doesn't take into account boost & special moves, though, which you can walk into the fight with stocked, and probably should count for someone like Kevin or T-elos, although it's questionable for Shion. If we assume "support credit HP" and "boost/special moves discredit" cancel out, though, this implies Shion's Medica S heals a mighty 1392/18188 = 7.6% PCHP (!!). Two Shiho bird thwacks do 7.4% PCHP (!!).
So, from the numbers in the topic, it's almost a draw on long term damage - Shion deals 6.9% PCHP damage every two turns via damage + heal, Shiho usually spams 7.4% PCHP with 2 bird bashses but occasionally has to give up a turn for a much stronger full heal, meaning they're dealing equivalent damage. However!!1! This analysis ignores Break! Shion can tank her damage some to use Break Bash and break Shiho a lot. However, boss Shion's BP is *horrific*, and I certainly let non-XS3 PCs break XS3 bosses, so uh Shion's getting dizzied by birdies, too. The Break damage average per character is 65 (when maximizing break) and Shion sports an amazing 80 BP, so that's like 2 Bird Thwacks -> dizzy. Despite the fact that Shion would, ignoring break, barely stay afloat via healing every other turn to me (I wouldn't allow it any more frequently), I think *she's* the one who ends up broken too much and possibly break-locked by Shiho: Bird, Heal, Bird, Skip, Bird, Skip, Bird, Heal, Bird x2, etc. This might actually kill Shion. Vote changed, since the breaks don't seem to fall her way.
WTF.