I'm using ent_move for my high-speed arrow in my bow-and-arrow type of game, and sometimes it impacts a model(The enemy) and sometimes it doesn't. I'm guessing the problem is its high-speed, so would anyone know of a work-around for this? It sometimes just "flies" through.

Use c_move instead. C_move uses a complete diferent collision detection system and with that instruction you can use precise collision detection v.g. my.polygon=on.

Also i think ent_move will be removed from the engine in future updates and it uses a AABB system for collision which the bounding box dont rotate with the entity, so the collision isnt accurate.

With c_move you have the possibility to use a OBB - oriented bounding box (rotates with the entity) and the collision detection its much reliable.
Hope it helps.