CU has been great since they changed it. You should be using it frequently, not sparingly now that you need to channel it only for 1-3 seconds until the server ticks and applies the buff.
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I highly doubt it. With six mobs, a single holy is 1050 potency. It's virtually impossible for balance to out pace that. Not to mention while you're chain-casting holy, you don't need to do any healing for the first 10 seconds or so, and the fact that you can't consistently draw balance.
It's better then the gravity ast doesn't even have at that level. With the damage it stuns which is why holy is better.
While AST is certainly serviceable in 50 content, from a trash compacting standpoint you get significantly more mileage with Holy (or even DoTs + Bane from a SCH) versus an AST with the only AoE option being Blizzard II. WHMs make the fastest runs in 50 since their upfront DPS with both AoE and single target in max ilvl gear helps melt things excessively quickly with little concern for their MP.
WHM is my healer of choice for when I want a 50, 60, or Expert roulette done as quickly as possible since the AoE burst damage is so high.
I enjoy SCH and AST more, but there's something to be said for Holy having a stun and requiring no target, especially when you pair it with Assize and Aero III at 60. Trash compactor indeed.
Only advice I can give (Since others have covered the bulk of important tips.) In a pinch, Synastry+Nocturnal Sect+Aspected Helios can provide a good amount of breathing room since the shield blocks an amount of damage equal to 150% of the HP healed when the shield is put in place and Synastry ups your healing potency, so this will result in stronger shields. So if things are going south, hit this combo to cover your party with a good amount of EHP, and it will give you the breathing room to pop lightspeed and SPAM THE DAY LIGHTS OUT OF HELIOS!
Also remember to relax and have fun! A stressed healer can be a sloppy healer.
On a side note to any new ASTs who might not be aware, keep in mind that this isn't literally how Lightspeed works since it does not change your GCD at all, just the casting time.
It's still good for added mobility and MP cost reduction, but the only spells you can spam faster under its influence are those with a cast time greater than your GCD.