Melee multipliers, general example: 2x Doublestrike * 4x MP * 2x crit profile * 1.5 hits/second = 24x total scalars
Caster multipliers, general example: 14x total spellpower * 1.75x crit profile = 24.5x total scalars
So ballpark equivalent at comparable levels of investment (arguably caster requires more investment, PLs and such, to hit those values)
Caster damage (1d6+6 as an "average" spell damage for a combat rotation): 9.5 * 30 = 285
Base damage, melee: 6[1d8+3]+15 = 60, 45 Mainstat mod * 3 = 135, +17 Deadly/Ins Dead, 6 sneak die @ 300 MP = 173....all together 385 so far
And of course there's way more meat on the bone for boosting melee base damage (enhancement +damage, imbues, filigrees, higher-than-assumed values, etc.). Caster base damage, thats pretty much what it is...there's no flat boosts to base spell damage.
So the math suggests its the base values that are limiting caster damage the most. All you can do is try to squeeze more high-power spells into your rotation, to boost your average. But there's a limited ceiling to how many things you can add before you need to start filling it out with 1d6+3 SLAs and other "filler". Especially since -% cooldown doesnt apply to SLAs, which includes all Destiny abilities.