BC should easily have superior speeds to BB, that's what historically made them a Battlecruiser and not a Battleship. The purpose of a battlecruiser, if I may quote a very famous man, was to float like a butterfly and sting like a bee. It could only achieve this with its enhanced speed.
Historically, the battlecruiser and the battleship eventually merged into the same class as engineers figured ways to propulse the larger and more heavily armed battleships as fast as the batlecruisers to the point where the latter became irrelevant. HOI2 does not address this well with every level of battlecruiser having just about the same speed as the same level battleship.
could you give me historical example of BC, that was significantly faster than BB? (the same generation, of course)