These are quality of life changes I'd like to see (on RDM at least). Because Impact is just a better Jolt, it could replace it like Ruin IV replaces Ruin II on SMN. And if Impactful falls off during your cast, you should not be interrupted either.

A quick tip is to never let impact fall off. So if you have been lucky enough with procs, and Impact is around 5sec remaining, use it anyway (or even before, if you go in your melee combo, to unbalance your mana and proc on Ver Holy/Flare).
Say you have balanced mana around 40 each, VerFire and VerAero both ready and around 20 seconds each, and Impactful falling in 5seconds, use Impact and dual cast your Verthunder if black is lower, VerAero if white is lower. In the best case, it did not proc, and you can go back on VerStone or VerFire. In the worst case, you will refresh that timer, while keeping an even potency.
Losing Impact and thus using Jolt II later is a slight DPS loss.