It simply depends on whether the concern should be proportion relative to one's role alone, or to raw ("effective") increased contribution to one's party.
Between any two Savage clears, there's little to no difference in indirect contribution (i.e. healer GCDs saved via mitigation) between two players on the same tank; with few exceptions there is generally a larger difference due simply to passively granted eHP (from gearing) allowing for more efficient healing (via delay) than between any two acceptable CD schedules, and it's not much either. That leaves the extent of added contribution mostly under the damage dealt metrics. But for two roles to be equally rewarding or mastery, they can't just have equal % gain relative to themselves, but also to each other. Otherwise, if you have a player who can perform at the same high percentile in either role or a given job in either, the one with the lower ceiling would technically be holding the party back if there's a lower-performing player when assigning their better player to that role relative to another, lower-performing player (again, on both roles or jobs therein). That circumstance would be so rare that all this is theoretical, but that doesn't mean it shouldn't be a concern.
For my own part, that just means there should be more tank control over their mitigation -- whether that be by sharing uptime cost with maximal damage or simply having finer means of control and, in either case, more effort necessary to optimize its potential, with a higher ceiling and lower floor so that tank contribution over percentiles scales more equally to DPS contribution over contribution rather than just in equal proportion. There should be an economy to it all, imo, whereby an equal increase in player skill in any given role is never going to be worth more in one role than another.
At most, the only exceptions would be to the extent that (1) the given encounter requires and rewards that increase in skill more in one role than another, such as in a fight with advanced tank-dependent techniques for uptime, or (2) the given job in one role as compared to another's better suits that improvement in terms of its skill curve. But these are both negligible concerns and we cannot afford to reshape job balance on a fight-by-fight basis, nor should we sacrifice originality in job design just to keep their skill curves equal.