Based on the way S-E designed AST, I feel like AST was designed to be a single target healer (more so than SCH now) since they don't have anything particularly bursty heals in the AoE department compared to Assize and Indomitability. Synastry is very limited compared to more universal buffs like Divine Seal and Fey Illumination. Also, I don't really have any concerns with ASTs base potency values as they're already covered via faster output (Diurnal) or more power (Nocturnal). As many others have commented, I'm also more concerned about ASTs ability to pull more healing out on demand like SCHs and WHMs can. So, let's build on that a little and see if we can make something comparable to what WHM and SCH has since both have good "o crap, I need to boost my healing abilities" while maintaining the unique flavour of "fast, single target heals".
Synastry - 20s of 50% healing potency when healing someone who isn't buffed by Synastry on 120 second cooldown. Effectively 8.33% increase but limited by the fact it's only affected by Benefic / Benefic II / Aspected Benefic. While the potency value is great, this is a really restrictive spell and if you're taking the time to single heal your entire 8 man up, you run the risk of someone eating danger and dying. I feel like the simplest change is just allow the buffed target to get 50% of the healed amount regardless of if the AST is healing a Synastry'd target or not. May need to tone down the potency a little to suit, but this gives AST a viable healing booster that's still unique to their play style and much more usable.
Lightspeed - As many others have said on these forums, a GCD reduction would go great with this. 25% would be nice and that'll allow you to fit six spells in what was previous a four spell period.
I feel like that'll put AST on track for emergency buttons. Essential Dignity is already really powerful and doesn't really need to be adjusted in any way short of a very slight reduction in its' recast time.