Without that change, heavy battleships would simply be better in practically every way than lighter battleships. Pretty sure it's necessary unless you feel heavy battleships are severely underpowered right now.
Compared to light battleships they truly are. I dont think the agility reduction is necessary in order for the speed boost to be truly effective. And if it is then a lower amount like say 10-15% would be better.
You are right in saying that battleship vs battleship combat without a cder is pointless however, so such matchups shouldnt take place