it cannot fittingly be described as a form of energy because it is the substance of an extradimensional space beyond the "jurisdiction" of physical law
... I'd say that's up for debate. Energy is something surprisingly hard to define - we define it by what it can do, because there is no real "be" about it. It is only indirectly observable - you can't point, at any level of scale, and say "There. That's energy", you have to say "That is something that has energy".
It's usually defined at its simplest as something that makes "work" happen - that is, that it can do stuff. And by that yardstick, I'd say warp-stuff can be called energy, because it's very capable of doing things.
We'll agree to disagree here, but I can see your point. I've been working to an expanding theory of aethyric dynamics for some time now (the first points of the theory having been defined about a decade ago), which does heavily influence how I view the interaction of the warp and real-space.
In part, I tend to prefer the term 'warp-stuff', because it's not burdened by concepts linked to physical law, even if it is somewhat vague or ill-defined.
However, the common definition of necromancy referring to the zombie raising sorcerer type has obviously shifted the term.
Clearly, though many necromancers of fiction do at least partly fit the traditional definition by communing with spirits and gaining knowledge from the dead. A preferable term (and one far more laden with opportunities, IMO), is Thanaturgy, derived from
Thanatos and
Thaumaturgy.
And that... well, that seems more than a bit off as a sequence of events to me. Like I say, I'm not sold on the logic that energy/matter is not real because it was once warp energy.
I see your point, but there are a few specific differences in our interpretations of the situation here.
The energy and matter are real, but the Warp is permitting 'impossible' interactions by circumventing physical law. A fireball conjured by a pyrokine is a ball of fire in every sense of the words... but the circumstances by which it comes to exist are artificial - eliminating the need for fuel or an initial heat source (or even oxygen). It's impossible fire. So long as the pyrokine is able to exert influence upon it, he can continue to permit the fire to exist... but the moment that he ceases exerting influence, the fireball once more requires all the things that fire needs to exist. Under normal circumstances, this means that the fireball will dissipate - it has no fuel to continue burning.
To go to your biokinetic example, the warp-spawned effect is the abnormal stimulation of tissue growth, rather than the tissue itself. It would be the path of least resistance (tissue growth is a natural process - the unnatural part is the speed and artifically-directed nature of the growth). Once grown, the arm is entirely real - if it wasn't, it would require continual concentration to maintain (loss of concentration when controlling "warp-stuff" is what causes psychic phenomena and perils of the warp), no different to injuries caused by psychic effects being real (a telekine can break your bones - his method is unnatural, but the injuries are real).
In essence, the effect of an active psychic power is to bend or outright ignore the natural laws of the universe (a passive power is, as previously noted, defined by observation of the Warp to glean some supernatural insight - according to Gav Thorpe's
Path of the Seer, the Eldar seem to regard this as the fundamental heart of learning psychic powers, and their focus on mastery of those techniques is why they are so skilled within divination). Different psykers, beyond their innate capacity to draw from the Warp and influence its use, are more or less proficient at creating particular effects based on their own knowledge and instincts.
