The difference in speeds would yield a difference in kinetic energy that is too small to notice in terms of the damage caused.
Yep, little difference in kinetic energy. In fact, the bomber's
rear firing guns will always possess a distinct advantage in impact energy (against a trailing target), as well as retain their energy better (vs range), due to airspeed relative to the rounds while in flight.
Moreover, cannon armed aircraft inflict most of their damage with chemical, not kinetic, energy.
Compared to MG's, think of cannon firing HE as more like "grenade launchers" than guns. Except for the Mk 108, which is more like throwing the grenade by hand.