Cast Time doesn't work like you think. Spells take 1/(1 - Cast Time) as long as their nominal time to cast. -100% Cast Time thus means shooting off "only" twice as many spells in the same length of time as without any CT. If you add up your Cast Time enchants and ignore the minus sign, that's the improvement in your spell DPS, assuming that you're spamming a damage spell.
The CT enchant is only available on gloves and staves, and the strongest CT enchant is about 63%, so you'll top out at around 226% of unmodified spell DPS. The items tend to require around one character level for every percent of CT, too. You won't see 44% and up CT gear for quite some time.