What makes WHM special? Nothing anymore.

[Jabtangs]

Fast Heals (Single and AoE): While WHM has PoM (20% speed, 15 sec, 150 sec CD), AST's Lightspeed is 25% MP reduction AND a FLAT 2.5 sec cast time reduction for 10 seconds (making most spells instant or near instant) and is ALSO on a 150 sec CD. This lets AST imitate a burst AoE heal combo like Medica+Assize by simply alternating Helios.

would like to know how instant casting a heal every GCD makes it 'faster heals' compared to a skill that makes the actual gcd shorter

Party Buffs: AST
DPS Contribution: AST
MP Management: AST

Unreliable buffs don't count towards having a higher edge on the matter.

Also, enmity management is another thing that should be on the list. Probably more stuff to be compared as well.