I don't think psychic vampirism is a path we want to go down. I shouldn't need to explain why feeding our enemies to Slaanesh is a terrible idea.
And once we have the soul connection removed, assuming the ability remains, i'd still be hesitant to use it at all.
"Isha would approve" is imo a cop out.
Even if predation falls under her domain (of life), most stories about her portray her as a gentle being.
Unless that gentleness was an affectation, she probably would not approve of psychic vampirism on a wide scale.
But rather than arguing against it based the true nature of Isha, i'm going to argue based on the nature of the Eldar.
Eldar are not known for their impulse control. It might start out as "skimming a little off the top" but it probably wouldn't stay that way for very long. Who is going to police it? This is exactly the kind of thing that Eldar would get addicted to.
And if it' turns out it's a natural feature of the Eldar soul post fall, and not something Slaanesh induced, I would probably vote to remove that in the soulforging process. Being less like She Who Thirsts, should weaken her claim further.
The other deal breaker is that this is the kind of dominion level shit that gets you an even worse reputation than we already have.
And if using other races as catspaws to fight battles for you (even if it's a common enemy) gets you a reputation for untrustworthy perfidiousness, regularly using other races for food gets you on everybody's kill lists.
And we don't have an unassailable fortress and plot armor to get away with it either.
When a Dark Eldar eats the emotion emitted by someone, they're not feeding that person to Slaneesh.
Firstly, they don't have to hurt people to feed on the emotion they emit. A Dark Eldar can feed on any emotion. It's just that the stronger the emotion the better, and it's easy to induce people to produce strong emotions by hurting them. A dark Eldar can feed on the energy emitted by an audience for a performance if they're into it enough though; there's at least one example of one doing so. That doesn't appear to hurt the audience in any way. It's just easier and quicker to use pain to make people suffer than to use humour to make them laugh or inspire religious devotion to make them sincerely pray.
Secondly, the emotional energy they eat isn't fed directly to Slaneesh. It's absorbed by the Dark Eldar. They can do a variety of things with that energy. They can use it to heal their injuries. They can use it to reverse their aging. They can use it to supercharge their physical abilities. And they can also use it to recharge their souls of energy that Slaneesh's Thirst has drained.
On Isha: she's the god of the harvest, of agriculture. The core of that is about eating things, usually if not always after killing them, and that applies to slaughtering the herds as well as reaping the crops. She may not be her husband, god of the hunt, who is about actual predation, but harvesting energy is the very centre of her portfolio as Goddess of the Harvest. If it's part of her Blessings I would assume that the Old Ones built it in as part of weaponising Isha's Harvest Goddess portfolio against the pre-biotransference Nexrontyr. It's an obvious way to turn the nature of a deity of agriculture and the harvest to martial purposes.
Havesting energy like this doesn't seem to have much to do with Slaneesh's themes, any more than any activity that you can over-indulge in does, like the desire for perfection when crafting or performing art, or the desire for sensation when experiencing anything. Just like everything the Eldar do in this era, it would need to be done with discipline, but that's no different to literally ever single other action or thought.
The Eldar can decide to commit atrocities for any number of reasons, pragmatic or not. Having the extra option to use atrocities to generate extra emotional energy is just one more on top of many. If they weren't prone to atrocities in the first place, I don't see that being able to do things like feed from the energy of passionate sports or music fans or from religious devotion makes them so much more likely to.
Removing the ability to harvest emotional energy in fear that Eldar will overindulge in that seems like removing the ability to eat regular food for fear of getting fat, or their ability to appreciate beauty for fear of being lost in pursuit of it.
Carving out more of the already damaged soul and removing part of what may be one of their few surviving connections to one of their few surviving gods seems like a bad idea to me. If it is part of Isha's blessing I think that connection would help and Eldar resist Slaneesh, as they're acting in the role of a deity that hasn't been assimilated into Slaneesh.
In quest, if this is the case, this could be why the Dark Eldar can get away with this, as Isha is not now part of Slaneesh so acting in line with her portfolio by harvesting cannot be in line with She-Who-Thirsts, as Isha is an discrete deity who still owns her own portfolio, and to the degree it's compromised it's Nurgle who is encroaching on it, not Slaneesh.