Have both, see no difference, and 51 still tanks and dps' like a boss for me. But maybe that's because it takes all of 5-10 seconds for me to clear mobs.
As for the 51 mission "soul breakingly tedious" is opinionated. I happen to LOVE it, and have done it more times than anything in KotFE. But since 55 coming back needed a qualifying sub date then the only other option is 51. Just because someone wasn't subbed by a certain doesn't mean BW needs to make it available to everyone.