I think the question of what looks good on the model is the more important one, but, for what it's worth, some SFB history:
The War Cruisers (CW) came into being in Volume 2 (1984), and were described as "the cheaper, faster production units with shorter lifespans." Later, the descriptive term "hot warp" was added, maybe in Rules Update #2 (88), to clarify that CAs couldn't have hot warp engines (and the above-combat speed power they provided. When there was enough of a demand, ADB relented and released the Heavy Cruisers with "fast warp," the CFs, in Module R6, with this description:
--THE FAST CRUISERS: These ships, an experiment from the dawn of the General War that was not repeated,
sacrificed some of their heavy weapons for increased speed. They were an outgrowth of the same "hot warp" technology
that yielded the war cruisers and war destroyers - ships with more warp power than was needed for top tactical speeds.
Experiments proved, however, that the hulls could not stand the shock of a full heavy weapons load in addition to the
"hot warp" drive. These ships were originally considered an alternative cruiser design, then became a "special mission"
type used for raids and reconnaissance missions. [-The concept of "fast cruisers" was originated by Stephen Latus]
So, "hot" warp and "fast" warp are closely related but with different uses. You could make the argument that the "CFs that could withstand the shock" became known as X-ships.
The model of the NCL just went through a design change - the Starline version just had nacelles that were too small. I have one sitting in a diorama on my desk, and every day I say I have to get new nacelles someday...
One more observation - the nacelle size ratios Bernard Guignard listed just seem to be 10.24 meters per warp box. 10.24x6=61.44. 10.24x12=122.8, etc. To me, it's the whole NCL that has 2/3 the mass of a CA, but that doesn't necessarily apply directly to the parts.
For what it's worth.