Well, "the Japanese" is a bit of a generalization here: the Army always felt that the best way to wage WWII was to send untrained peasants with bamboo spears to hack down inferior foreign scum, whereas the Navy and the Air Arm steadfastly believed in high tech but were starved of resources to build it, and towards the end of the war had to concede that suicide workers were better than the pitiful automation they could come up with in the circumstances.
Romulans in the Romulan War as described in the ENT novels would not appear to be starved of industrial resources or reliable automation. Quite the opposite, the Romulans there are the galactic masters of remote control and uncrewed combat. A crewed suicide craft would thus be more likely to fail than an uncrewed one, especially if confronted with the unexpected.
The stories take a convoluted twist or two to negate this almost canonical advantage of the Romulan Star Empire, but they fall short of turning the Romulans into WWII Japanese here...
If the starship derives its energy from its store of antimatter, it doesn't matter how fast the ship is moving when it hits a planet, just so long as it hits the planet with sufficient force to rupture its antimatter containment system.
The two advantages high warp would offer in a ramming attack would be
1) reduced chance of intercept because of reduced time within range of defenses, and
2) possible deeper penetration of the warhead, not so much due to kinetic energy (which may indeed be "virtual" at warp) but due to the reduced interaction between a warping ship and the universe around it (either because the ship is "in subspace" to a greater or lesser degree, or because of the devices and techniques the ship must be employing to make herself invulnerable or invisible to interstellar matter).