That's not quite correct.
Haste's effect on cast times can be expressed as: base cast time / (1 + haste). Haste's effect on mana regen can be expressed as: (mana / time) * (1 + haste). So, the full formula for the amount of mana you'd generate per a given cast time would be: (base cast time / (1 + haste)) * ((mana / time) * (1 + haste)). That "1 + haste" cancels out, so you'd end up with: base cast time * mana / time.
Or, a given casted spell will generate the same amount of mana no matter what your haste is. So, a Chaos Bolt will always generate 21.75% mana over the duration of it's cast (barring the use of Backdraft, which is a cast time reduction instead of a haste effect).
With one big exception in the GCD cap. Once you hit the 1 second GCD cap, your cast time can't be reduced any further, so you start generating more mana per cast. One hidden and often overlooked effect of our Chaotic Energy passive is that it reduces the GCD of our spells by .5 seconds, so all of our instants have a base 1 second GCD without haste.
So, as you get more haste, the mana you generate while using Congflag, Shadowburn, FF, and Curses will increase beyond the base 7.25%.