Yeah I got the standard 0/9/21 support page, just messing around with a sorta less squishy Sona thing 
If I'm running exhaust support, is it worth it to ditch Scars and go 1/8/21 or should I just forget the penetration boost and stay 0/9/21?
i would say it largely depends on your lane partner and character
like if i was playing leona, for example, and my partner was varus, i would consider going 1/8 instead for the penetration because he lacks any naturally
but if my partner was corki, i'd probably go 0/9 because corki's passive and gatling gun's armor shred make the mastery less useful.
there's also things to consider like how much hp synergizes with defensive buffs(taric's shatter, leona's eclipse). 30 hp might not seem like a lot, but it becomes a much higher amount of effective hp when you have on demand resists.
the point for the mastery is more attractive for lanes like ashe/nunu, where neither of them have any defensive buffs or shreds, but less attractive for lanes like corki/leona, where there is shred naturally in the lane and you have a defense steroid to increase hp effectiveness
also as an aside, it'd be more effective to mention the stat boost of the mastery, since no one knows the mastery names off hand really. i had to look up which mastery you meant rofl