The problem with double mana cost for ele is that casters never go oom. I dont think so that would be a too high costs for them.
Except that doesn't matter for Ele because it is something that they need as well for offensive utility, and it is balanced because Elemental has only shocks for instant damage. All their other damage functions are casted, unlike what most of the other casters have to do now. So for them to spam purge casts means they're not doing damage. If they're getting trained to the point where they can't get much if any casting off then being able to clean a target means they're still helping offensively, and if they're not getting trained then they're not going to be purge spamming outside of big buf times because then they'll be casting their damage spells.
The extra mana cost is only for Enhance to have to use it smartly, since they can still auto attack and get weapon procs while spamming purge, not to mention then using instant melee attacks.
As for Ele Blast being too strong as instant, well thats why i said 0.5 sec too, so that they would have to stutter stop to use it. But even then I don't think it'd be too strong because if it moved to instant it would get less spellpower and thus not hit as hard as it does now. Regardless it needs something substantial, because just the extra buffs it provides simply isn't enough for it to be taken over UF.
Remember that any Maelstrom used offensively for UF could have been a Lightning Bolt anyways, so subtract the dmg of a regular lightning bolt and the "extra" damage isn't as large as one would assume. On top of that, most Maelstroms need to be used for healing or held for an upcoming Hex. Lightning Bolt is rarely more than 10% of my damage, if it even comes close to that (i'm thinking 5%, i'll have to check my logs next time I do a session). In PvE, UF outdamages EB straight up, if not by much, it still does, and you have to remember that is with constant uptime and all Maelstroms used for EB and lots of Maelstroms from the 100% uptime. The damage can't be out of this world because it is an uncounterable instant attack, as much as I'd love to see it hitting for 90k.....it just would get nerfed right away.
So what Ele Blast mainly needs is a way to cast it that doesn't involve sacrificing the defense/utility of Malestrom healing so that it can be frequently cast to get the damage output in PvP to be worth taking over UF. Keep in mind you don't just get sustained dmg from UF, you get the sprint mobility from FrB. Sustained and utlity vs burst. But if the burst can't happen/isn't worth it, then it is no choice at all, just like how the talents are now.