Lower warp speed that doesn't matter in a fight and that wouldn't be used that much anyway as everything needs to keep pace with the Thunderchild during fleet movements.
Lower firepower that we probably won't pick up anyway as the whole point of this design is for it to be constantly pointed toward the enemy.
*delurking*
It matters for the conduct of the war.
It means that Starfleet can send NX-class cruisers on scouting missions and deep strike attacks against Romulan logistics bases and supply ships with an escort of Selachiis that can actually keep up with them at maximum cruise, making for bigger raiding forces that the Romulans have to react to in the deployment of their warships, putting them on the defensive.
It means that task forces composed entirely of Project Selachii ships can probe forward into the conflict zone and evade pursuit at will, forcing adverse deployment decisions on the Romulans.
It means that you can cover friendly space and your own logistics lines with fewer Selachii-class patrol ships, freeing up additional forces for offensive operations.
All of this forces strategic changes on a Romulan force that is currently largely composed of Warp 3 ships.
That kind of pressure shaping the enemy response is how you win wars.
Remember, you currently only have one Thunderchild at the moment.
Meantime, Starfleet has been building an NX and five Stingrays every year, a build tempo that is likely to be sustained or increase with the Selachiis.
The Thunderchild matters for major fleet engagements, but in the day to day cut and thrust that makes up the majority of military operations, its the NXs and the frigates that you'll see.
EDIT
Just so its clear, Im advocating that we vote for
[]External Nacelles
The strategic speed matters.