I think it should be made instant anyway.
Nerf the heal or not, I don't care, but as a sustain mechanic an instant cast exchange would really make sorc builds significantly better in PvE.
With half the resources returned over time, it would function in a very similar way to siphon strike or the betty netch.

I was looking into whether this skill would be worth using for sustain after the latest changes on the PTS. It returns just under 6k Magicka over 20s. Since most Sorc rotations are either 8 or 12s, it would likely be cast every 24s, which results in 250 Mag/s.

To make use of this ability costs 1 bar slot, 2 GCD’s, and 1 Light Attack. The bar slot is assumed to be on the back bar, and likely something like Inner Light or Bound Aegis is replaced. Both of these skills passively increase damage by about 4%, while Dark Exchange only passively increases it by 1% from the expert mage passive, resulting in a net loss of 3% DPS while on that bar. With a conservative estimate of 1/3 of damage being done while on back bar we can call this a 1% DPS loss just for slotting Dark Exchange.

Now by casting it you are also giving up 2 spammables every 24s. This could be something like Force Pulse or Elemental Weapon which average around 16k damage, or 1/4 of the time it could be a Crystal Frag which hits for about 25k average. We’ll go with 18k average for a spammable, so 36k damage is lost by missing two to use Dark Exchange. You also miss one light attack which casting, which is about another 12k damage. So 48k damage lost every 24s, which is a 2k DPS loss.

Combining these numbers, we can say that the 1% loss from slotting it is about 500 DPS (for a 50k DPS parse) and casting it is another 2k. So total 2.5k DPS is lost to gain the 250 Magicka per second from Dark Exchange, or 5% DPS loss.

Now I want to compare this to a different source of Magicka regen, jewelry enchants. Each gold regen glyph gives 169 regen before buffs. This is modified by a multiplier that is typically about 1.85 for an Altmer Vampire with Major and Minor Intellect, bringing the regen of each glyph to 312. By using two of these enchants you can get 625 Regen, or 312 Magicka Restored per second. This is 62 more Magicka per second than Dark Exchange gives based on the numbers above. To get this regen you are giving up two Spell damage glyphs, each which reduces DPS by about 2.5% (assuming raid buffs and two damage sets), so net DPS loss of 5%.

So comparing these two methods of building for sustain, it is clear that Dark Exchange is still inferior in resource return (62 less mag/s), drains your stamina (allowing less blocking and dodging) makes for a more difficult rotation with all the risks associated with using a cast time ability, and provides no benefits compared to just adding more regen at the cost of spell damage. This is why Dark Exchange is a dead skill for PVE Sorcerers.

Hypothetically, if it was made to be an instant cast skill it would only take one GCD, and no light attacks would be missed. This brings the DPS loss of using Dark Exchange down to 2.5%. With this change it would be fine to reduce the total Magicka restored back down to ~4800, and even shift most of it to the end of the duration (maybe 1200 instant, 3600 after 20s) to prevent it from being spammed. This would only end up being 200 Mag/s if used in a 24s rotation, but would be less risky than a cast time and fit better into a rotation.