Bah. I never use tactical. With tactical on you can easily wipe out vastly superior AI forces with greatly disadvantaged force of your own. I don't like picking on the retarded AI so I give them a fighting chance and come up with my own strategies in strategic combat mode. It also makes great practice for PBW games.
And for the speed v power formula you really should go with Battleships. The single point in strategic movement that is lost is negligable while the gain you get in firepower from the next strongest level of weapon mount is great. I used to go the BC route as well but it just isn't effective late game against humans using Battleships and larger.
PS: The fact that the AI was using projectile weapons on DNs proves the point that it really isn't the sharpest knife in the lot. Where you using TDM or just standard? In my experience the better custom AIs all break formations in combat.