It has nothing to do with the fuel tank. The -4 has four (4) blades in its prop and the -C has four (4) barrels in its wings. The tempest has both so it is perked higher.

As for what Karnak said is correct, they are perked for a reason as they have a very high capability/impact on the game. The Dash 4 is one of the finest aircraft in game, quite easily can catch most planes running, and out turn most planes.
When you look at the P51D vs Dash 4 on paper, the Dash4 is three times more capable to handle any situation or role in game where a P51D might not be as successful.
Chog simply is a 4 cannon monster, while not as fast as a Dash 4, it carries 924 rounds of 20mm which gives it tremendous firepower as well as capability, as for a fighter it can man handle most planes in the game if flown correctly.