I would like to see parts form the Heavy Escort Akira and its varients added to the Steamrunner. Specifically the option to choose the nacelles, the deflector, and possibly the saucer. Obviously the plyons would not be customizable, but steamrunner unique pylons could be rendered.
I bet the real reason the Steamrunner won't get customization is because of the way it's nacelles are attached to the saucer. To make the nacelles usable on other ships, they would have to model the nacelle parts hidden by the saucer. To add other nacelles to the Steamrunner would require making them look good under the saucer, they would have to do more textures in the areas in and around the nacelle housings too. It's a heap of work we will likely never see, but still you have my support, this would be wonderful.
The Somraw, K'tinga, D'Kyr, D7, Kumari, Xindi carrier, Xindi escort, and the T'Varo are all older than the Constitution Refit and yet they are tier 5. The Constellation is made up primarily of Connie refit parts and it is tier 5, there is no logical reason whatsoever for the no tier 5/6 connie rule.
I think the key is the encoding that specifies where each part is attached to the others. A good way to see this is to look at each part comination (think Nth equation). After doing this, it becomes easy to see why the Steamrunner and the Norway are difficult to kitbash with the Akira. Therefore, I recommend that only the parts that are easy to carry over, such as the deflector dish and the nacelles are introduced and the other parts are added as the encoding improves.
That's what the Oslo (or was it Zephyr?) design on the Akira class is for.
This. Oslo and Zephyr were introduced as Akira variants because in the time allotted for designing the ships, the devs couldn't find a way to make the Akira, Norway, and Steamrunner parts play nice with each other.
In my opinion, that's just fine. The game is already rife with horrifying kitbash potential, I don't feel the need to see more bad kitbashes...