Heh, a couple of weeks ago I was talking to a friend about Sins and we brainstormed our "dream" fourth faction.  Our dream faction had a superweapon similar to this, opening a temporary one-way wormhole to any planet which you had units.  Didn't know the Kostura was actually getting changed to have this effect, but it should be interesting to see how it planned out, as we actually argued over this one concept for a while.

I'd say that opening a phase line to a target planet for invasion is powerful enough as a superweapon on its own, especially for the Vasari who get phase stabilizers.  The Kostura should probably lose its original effect if it's gaining this one.  It just seems wickedly powerful, and unless we're talking about a small map (in which case superweapons won't come into play very often!) far better than any of the existing superweapons.

And people are saying Varsari are getting the short stick in this expansion.

Well, superweapons aren't exactly an every game occurrence.  Doesn't matter if your faction has a killer superweapon if the game almost never lasts long enough for you to use it.  Certainly I hope Vasari get their fair share of new content throughout the course of the game, not just at the end.

Yes, the Vas Superweapon really comes into play in large maps...especially across multiple stars.

Originally, every ship in the well would be damaged.  This meant that in the late game, you'd have to turn off all your cannons just to keep them from obliterating your own fleet once you got to the last couple planets.  In a patch (1.05?) they changed that; now, friendly vessels (yours, allies, cease fires) aren't stunned or harmed at all.  It's VERY nice, especially when the shell lands right in the middle of a large fleet battle.

It depends on what you want out  of it.  Generally speaking yes, a kostura shell is at its most effective when used to directly support a fleet's assaults.  But, in my experience that has three major drawbacks:

1> Kosturas, after all the AM upgrades, can fire as often as their recharge allows.  If you're holding them in reserve until the fleet is ready to attack the next planet, that's a lot of wasted potential; they should be firing as often as they possibly can, because even if they hit the "wrong" system, the stun effect will still be useful to you (economic disruption, slows down fleets passing through).  The damage is also nice; even if it doesn't actually kill any structures, any fleets passing through will come out half-destroyed.  I've had more than once where a massive enemy fleet attacked my planet with all of its ships already at 50% hull and no shields, making it very easy to wipe them out with an inferior force.

2> The things rotate and fire so slowly that it's very easy to get a wasted shot if you try to time it to land the shell in the middle of a battle; it could easily arrive after everything in the well is dead.  Leading off with them is fine (i.e., waiting until you see the shell in transit before giving the fleet the movement orders), but anything more precise than that gets VERY iffy, especially on large/multi-star maps.  Waiting for the shell to begin your battle also means you're not attacking as quickly as you could be, giving your enemy more time to rebuild fleets and place static defenses; I get better results from continuing to charge forward and hitting your foe before he's had a chance to recover from the previous planet's beating.

3> The micromanagement needed just isn't really worth it, outside of a few very specific situations.  In my opinion you're better off using that time to manage your fleets more precisely.  On a very small map, when you'll only have 1-2 cannons, no big deal.  On a large map where I might have a dozen or more, you just don't have the time.

So, I generally only hold two or three cannons in reserve for direct support (and to open phase paths for my fleets), and set the rest to autofire.  It's great against the AI, and not bad against humans (as you're now less predictable; they don't know which planet is going to go offline for 15s).