The Defiant was scary for what it was. As someone above said it was a combat runabout, except that it could probably stand up confidently to even something like an Ambassador or Intrepid class ship. Problem was it did that by stripping out all pretty much every diplomatic and science ammenity that had been common to Starfleet ships since at least Kirk's time, and using a very inefficiant warp configuration. The biggest thing we've seen them go up against was that Jem'Hadar battleship which was twice the size of a Galaxy class ship. And while the Valiant got blown to bits in that particular engagement, I'd say it made a damn fine showing of itself, given it was basically the equivalent to a guy in a rowboat with an RPG attacking the U.S.S. Missouri. The only time we see a Defiant go down hard without putting up any real fight was in First Contact against the Borg, and Worf was in command that time so it's kind of to be expected. lol
The Prometheus and the Akira were pretty much on equal footing, I think, the Akira seemed to focus more on brute strength while the Prometheus relied more on advanced technology.
If I was going to reshuffle the escorts at all the only changes I think I'd make would be to swap the Hermes/Gryphon/Dervish with the Akira & variants.
But of course, the reason they didn't do that is because they wanted a canon ship at every tier, and since the Norway & Steamrunner were kind of ugly and under-represented little things, while the Akira at least had some air time outside First Contact, and they already had the Prometheus for T5, it fit nicely into T3 as a companion to the Cheyenne and Olympic.