Next vote is on the injectors, I imagine given it's a compatibility option it'll probably relate to the size/complexity of them and thus the flow rate (power in effect)
As systems become more complex and thus sensitive to permutations in environment the extra space is likely going to be needed to push monitoring and control systems nessesitating a refit grade change anyway if just further down the line.
While that tech will eventually shrink the extra space can be used for all sorts of things like redundancy's and backups of components and systems so it's future proofing for further prototyping in future too.
Edit: Also with the far greater power older ships will need a refit anyway of their power systems so they can handle the increased loads so the oldest or those with the most complicated load balancing will be in the same position.
Edit Edit: Also if there are any complications in implementing the next desired changes then the extra space could be key in overcoming them
Reorienting the core allows the new constrictors to be installed, turning the new engine into an eight-deck behemoth, though still shorter than the horizontal version. Given that space is more constrained vertically in most designs, there is an understandable push to reduce the length as much as possible. That brings you to the injectors, which have already benefited from the new constrictors. To make further progress, you will need to overhaul the entire mechanism.
Which is harder than it sounds. Every starship requires the injectors to provide antimatter and matter into the reaction chamber, but different designs have been used over time. In the warp 7 engine the matter stream is injected directly into the antimatter release at a high velocity, and is then funnelled through the chamber via magnetic constrictors. Because the collision of the two fuels is guaranteed by their geometry, these can accommodate high throughputs and power loads.
The high-flow regime is still broadly applicable. With the new central reaction chamber both the deuterium and anti-deuterium are being accelerated by the new injector assemblies, which enables higher throughputs by default. But new injector geometries that take advantage of the greater available density provided by the new constrictors could further accelerate the fuel and increase available power. These new high-performance injector systems promise to be able to supply the plasma temperatures that will be required by future nacelle designs.
But there is a catch. Current EPS systems and warp plasma conduits are simply not rated to contain and regulate the full output of a warp core using the new injectors. That means a ship using the new injectors will have to have a redesigned EPS system with new materials, interlocks, relays…that's not something you can fix after the fact. Any ship wanting to use a Warp 8 Engine will need to have been built from the ground up with that in mind.
On one hand, a reduced warp core size and future compatibility with new nacelles that can leverage the engine's maximum performance. On the other, no back-installation on existing starships. It's not an easy choice to make.
[ ] High-Flow Injector System
[ ] High-Performance Injector System (Size Reduction) (Type-4 Nacelle: +Max Warp) (Refit -> Incompatible)
I'm repeating myself at this point, but I'm personally inclined to go all in on the new components. The Klingons are already rolling out stuff that's better than what Starfleet can bring to the table in terms of sustained warp factor, and you can bet they're just as capable of implementing incremental improvements to keep that edge as fast as we can close with them.
But there is a catch. Current EPS systems and warp plasma conduits are simply not rated to contain and regulate the full output of a warp core using the new injectors. That means a ship using the new injectors will have to have a redesigned EPS system with new materials, interlocks, relays…that's not something you can fix after the fact. Any ship wanting to use a Warp 8 Engine will need to have been built from the ground up with that in mind.
On one hand, a reduced warp core size and future compatibility with new nacelles that can leverage the engine's maximum performance. On the other, no back-installation on existing starships. It's not an easy choice to make.
Assuming I'm not reading too much into things (and haven't forgotten some stuff in the meantime), this will allow for us to provide more energy to our weapons and get around the whole tos style 'only fires from 1/2 mounts' at a time deal, which with our style of phasers should increase our DPS considerably. As well as the directly stated benefits here.
Please correct me if I'm wrong, but if I'm not this is a once in a design lifetime chance to take a massive leap forward, effectively decades.
Here's where an A/B design split would be nice, making the old stuff "good enough" whilst the new stuff can go ahead with the new geometry. I think we have to bite the bullet though, better to have a couple of decades with a hobbled fleet rather than having an inferior design for the entire lifespan of the nacelle.