Replying to myself:
In the Smoke and Mirrors ill controller guide the Tempest proc is not advised, but since that powerset doesn't feature end drain in general, it might not make sense. Or, the end drain is so small as to ignore it [like a chance to heal I had drop a while back].
The wording is certainly confusing, there. I *think* I know what it's doing, though:
The first line is the PvE effect. It does, as you surmised, indicate that the effect is a -13% endurance drain. The second line is the PvP effect (note the 'if target is a player' tag). Notice that in this case it says '-X endurance' rather than 'endurance -X' - comparing this to other endurance drain powers, we can see that this is the notation for draining a fixed number of points of end, rather than a percentage based drain.
So, in PvE, the proc will drain 13% end if it goes off, and in PvP, it'll drain 3.33 points of end. The proc has a 20% chance to go off, so on average, it'll drain 2.6% end in PvE and 0.66 end in PvP. Charged bolt has a 7% (PvE) or a 1.25 (PvP) end drain, so an endmod enhancement would net you an additional 2.33% (PvE) or .416 (PvP). For lightning bolt, the bonuses from an endmod are 3.33% (PvE) and 0.666 (PvP).
So, the times when the tempest proc wins are in PvE and PvP end drain on charged bolt. However, given that the endmod enhancement should also boost the end refund and would almost certainly be easier to get, I'd just go with the endmod enhancement - that is, if you bother with either one. Given the extremely small benefits either way, I would most likely fill the slot with more acc, dam, end, or rech, or just save the slot for somewhere more useful.
@MuonNeutrino
Student, Gamer, Altaholic, and future Astronomer.
This is what it means to be a tank!
Well, I hope I don't get those auctions then. 
Though they craft into good money, they just don't sell.
Hmm, if I do get the auctions, since they are available starting at 15 and have cheapy salvage, I might slot one and watch the "healing channel" and see what's up.
I really intend for my dude to be a sapper. And getting that going at low level is not happening well. By the time I pull it off they're dead.
I guess I'm just going to slot the powers like any other ranged damage. Fooey.
E^3s don't really get much extra punch out of their sapping at low levels because most enemies don't have the hitpoints to withstand more than a couple attacks in any case. If you want to get some visible use out of your sapping as a lowbie, try teaming up with another electric blaster or two; together, you can actually get some serious draining as soon as you get Short Circuit. Solo draining doesn't become as effective or useful until you get at least to the 20s or 30s with SO (or equivalent) slotting, and you'll need to wait until you get Power Sink at 35 to pair with Short Circuit to have a quick, reliable total-spawn drain at your fingertips without the assistance of a teammate.
Rule number six of an empathy defender is NEVER underestimate a blaster's ability to die. I don't care if he has CM, Fort, both RAs, bubbles (both FF and Sonic), and is fighting next to a Storm defender with hurricane on. If there is a way to die in that situation, the blaster will find it.
I can get +1 minions down to a sliver with SC now. At lv 19, I've slotted it with four lv22 EndMod SOs. They're 25%, so I'm capped. One Acc completes my 5 slot.
Once the tank gets in and get enough around, I'll fire off Ball Lightning, run in, SC, BL again, back up. If you look quick you'll see their sapped before they die.
The only thing that actually survives long enough for me to actually sap them is Edomites [is that what they're called...big Lost dudes...for some reason that sounds awfully Amish today] or Rectors, whatever, and CoT Mages.
I hadn't won the two bids I put in so no monetary loss. I'm always wanting to slot procs...they aren't even that amazing, but they're cool.
In almost all respects, Accuracy, Damage, and Recharge (in that order) are going to serve you better in those powers (Charged Bolts and Lightning Bolt). If you can make room for genuine EndMod enhancements, feel free. If you're going to hang any proc, I'd use the +Chance for Build-Up on Charged Bolts (in fact, I did it myself). Dead's better than drained, and the amount of drain we're talking about is so negligible as to be nonexistent.
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The question here is for an Electric/Electric/Electric Blaster, is is better to slot the proc or just an endmod in Charged Bolt or Lightning Bolt?
On Redtomax, I'm not understanding the effect numbers on the right:
http://coh.redtomax.com/data/powers/...fted_Tempest_D
Does it drain 13%, or 0.13 points? And does it indicate that if I am targeting through the tank, or some other melee, that number is 3.33 points...leading me to believe that it sure isn't 333%.
These powers are getting six slotted, so at higher levels hitting softcap isn't going to be a problem.
Thanks guys.