The difference will come from players learning the ins and outs of fights and learning what they can abuse, while something like the RDM has minimal room to go up even if they do learn everything about a particular encounter. Again, even if you nerfed RDM they will still be equal if not higher than the average player playing the most difficult job. For example bard is doing quite well in ex susano due to being able to move freely at all times, even with their amount of utility (which greatly outshines RDMs).

Even if you lowered RDM to 4k dps from 4.5k and BLM is up to 4.5k you'll still have fights where BLM only does 4k due to heavy movement while the RDM also manages 3.8-4k due to being punished less. These types of threads will then change to the real issue of "these jobs are punished too much to pull off their their maximum" which is the conversation I believe we should be having anyway.