No you do not - 2 reasons

1. Haste does not affect the GCD for melee attacks
2. Instant attacks do not suffer the delays related to waiting for the server to confirm that an attack has happened before moving onto the next sequence as the confirmation happens within in the global cooldown (unless your latency is really really horrible)

/castseqences work fine for melee rotations (though they can not accurately predict combo point gain) and at level 15 you do not have enough attacks to /castsequence. Maul is an on next swing ability that can be combined with any attack eg

/Cast Mangle (Bear)()
/Cast Maul

/Cast Lacerate
/Cast Maul

or even

/startattack
/castsequence reset=1 Mangle (Bear)(), Lacerate, Lacerate, Lacerate
/cast Maul

ps non of these macros are tested as my druids are boomkins while levelling but I did play a Bear tank in TBC so should be fairly accurate