That sounds about right. You inevitably still have to do some casted heals in prog or with lower item levels, which is personal dps lost, and buffs scale better on the other end with parties with better gearing and practice as well, so SGE/WHM should have significantly higher personal dps than the other two to make up for both cases.

I've also been thinking lately that AST's buffs being as good as they are should've only been allowed with the RNG. Have less consistency than WHM but higher output when the RNG's good, and have ways to minimize the lows of bad RNG with more effort.