Let the effect be 'allows you to take two standard actions this round, instead of one move action and one standard action'. Add on the usual mechanical bonuses from normal Haste.
Then, make it last for one round. Cast Improved Haste, then two attack spells. You get two spells a round for the cost of an extra spell instead of a feat. Mechanically, you get the effect of a Greater Metamagic Rod of Quickening, which costs 170k - something a 15th level character book-level wealth could afford, if he was saving up for it ever since he hit 7th level. Given that, my version of the spell sounds just right as a 9th level spell.
IRT Slaved:
3.0 Haste gives you two standard actions per round, so you end up being able to cast three spells per round with Quicken Spell. It gives wizards a LOT of extra tactical power, being able to pump out spells that quickly.