I'm a little confused over how the Engine enhancement synergy thing works. So, if you had a fleet with five FBBs and one BB with upraded speed, you wouldn't qualify for a certain brancing route if the requirement was 6 FFBs, but you would qualify if the requirement was a fast fleet?
Certain Map needs 4 Fast Ships and 2 Fast CV's. You want BB Mutsu who's slow to be in the fleet lineup. You can give her the turbine + boiler setup to make her qualified for the branching.
Certain Map needs 4 FBB's (Classification) and 2 CV's. You want BB Mutsu to be in the lineup. Its not possible as the branching needs 4 FBB's not a certain BB or BBV.
WIth that said, certain maps need speed classification and the turbine + Boiler setup can work, but if the map needs a class specification its a different story. Speed Classification and Ship Classification are two different things.