Mind blast is a 1.5 second cast time base. Starfire is a 3.5 second cast time base. Spell power coefficients go off base damage. There is no reason a 3.5 second base damage spell should dal the same as a 1.5. That is all.
Boomkins have a lot range and instant cast spells, shadow priests have no mobility. A boomkin can fully kite out a shadow priest and never let them get into silence range if they wanted to. So uh, yes it IS hard to get in that range.
An 8 yard range ability, a profession locked consumable, and a RNG based stun. My God, you must either never PVP or you play a boomkin. Holy Moly.