If two tanks have the same dps(t) curves, they have the same average rDPS. You obviously cannot assume the reverse, but that's not the point here. This is also a composition independent property for tanks, because rDPS = nDPS for them.

As a player who actually supplies raid buffs, if you have two tanks with the same dps(t) curves, they're also going to provide me with exact the same benefit. They both burst with the same intensity at the same time. I can take any buff application window buff(t) and reap numerically identical rDPS benefits from both tanks. This is the actual quantity of interest.

In short, if two tanks have the same dps(t) profiles, not only do they provide buff providers like myself with identical rDPS gains, but their own rDPS values will be identical. So no, improving a job's alignment with buffs doesn't have to come at the expense of rDPS parity.