In single target scenarios you shouldn't be that far behind WHM in DPS. Cards work off the global CD, so you shouldn't feel any real impact there. Regen is similar in power to aspected ben, same with helios and the generic cute/cure II spells. The mobility front load of light speed is powerful. Synastry is almost an on demand Benediction its entire duration (a crit makes it crazy.) It's subsidized mp costs make mp management less rng than a WHMs.

In my eyes, WHM will and should have it's niche cure III and higher personal dps, while ASTs have to flex a skill floor between not only healing and personal dps, but weaving cards as best they can to not interfere with their gcd. The best ASTs should be able to play as if uttering cards between the breath of each spell they cast, never really stopping for them.