My biggest difficulty with fusion as a power source for warp is that less than 1% of a fusion fuel's mass is turned into energy when burned. And in the case of single, uncatylized deuteium fusion, it's less than 0.1%. That translates into 100 - 1000 times the mass in fuel for the same range at any given warp factor compared to M/AM. And I'm not sure there's room for that much tankage even if we assume the nacelles of the BOP are the same size as those of Enterprise.
At the same time, the shape of the neutral zone --roughly linear-- makes no sense if you're encircling a single system, for which you'd want a sphere. And as has been stated earlier on this thread, an STL society poses no threar to an FTL Federation. Thus Romulan FTL is implied and the impulse power statement becomes one of fusion warp power.