It may be worth considering synergy, as well. For example, II ships work very well in fleets, despite being average on their own. A lot of players like throwing in something like an Interrex or Decurion with their fleets simply because they are reliable in the AI's hands.
Furthermore, a ship being
powerful doesn't necessarily make it a higher tier. Tier is a measure of
usefulness. An Olympus is powerful enough to wipe out a big chunk of the fleet, but isn't as useful as something like a Paragon, which is powerful while having the versatility and staying power to back it up. An Archbishop is extremely powerful, but its slow speed and huge logistics costs make it less than practical to use.
There is nothing wrong with having ships that lean towards a certain Tier, but that leaning must be justified. For the Templars, of course they're a boss faction, so it's a given. For the Imperium, having ships that are all solidly average is part of their persona. As you mentioned, the ease of acquisition is a major part of it.
Exigency ships are kind of special, by the way. They go up in tier if the player is able to outfit them freely and give them the benefit of full tech/combat aptitudes. A Zephos can go from a unique but vaguely above-average destroyer to a lightning-fast ninja of death capable of outperforming a Hyperion. An Irithia goes from a competitive combat carrier to a horseman of the apocalypse, especially with that absurd 50% missile damage boost.
More generally I don't think any ship with energy mounts can be considered top tier (at least when considering the vanilla weapons set).
- Dominator should be tier 3 or 4; extremely over costed hull - too little flux dissipation & OP to effectively utilise its mounts.
- Sunder -1 rank too; average speed, average shield, pathetic armour, ineffective mountings, AI is useless with it. Horrible ship.
- buffalo 1/2 not tier 5?
I suppose if their role is to die every battle, then they fit your tier definitions well 
Energy mounts get much better the larger they are. Large energy weapons are extremely good, and the Paragon has four of them. They all point in the same direction.
Dominator is fast (due to burn), has a nice array of weapons with good arcs, very tanky, and can dish out a ton of damage. It's even got THREE medium missiles, which gives it unparalleled burst damage. It might not have made Tier 2 without the buffs to missiles. It's also only 8 supplies/day, compared to something like the Aurora which is 10/day.
The Sunder is actually pretty dangerous if you outfit it correctly. It does a ton of damage very quickly. It would be Tier 2 if it were not for the fact that the AI is not very good at using it.
Buffalo are decent freighters. Not as good as Tarsus, but they carry just as much. The Buffalo Mk 2 would have been Tier 5 in 0.6.2, but the buffs to missiles make it really dangerous in a pinch.
