Is it more than 25pts better than the other engines of Vaul? Probably but that's because it fills a vital role for BT with the only non-spaceship barrage to hit the opponents deployment zone. Storm serpents+Cobras fulfil very difficult roles and only really fit in lists built especially around them and the Scorpion is in an AT role where there are other options available.
Against the right list - lower SR, small formations - especially artillery, they are really good. Against WE heavy armies or SM they are pretty average.
Good analysis Steve.
The Void Spinner has long been recognised as being slightly better than the other EoV, hence the 25 point points hike. I agree that should be sufficient for a single unit in the army.
The issue being raised is really about spamming Void Spinners, and whether they are more more effective in larger numbers. This is inherently linked to the triple retain - so in the worst case allowing the BT player to get three shots off against the enemy for no reply.
Put another way, in a game, assuming they get to shoot three times, are they worth 275, 550 or 825 points - will they destroy or disable an equivalent amount of enemy formations?
Obviously this depends on how they are used and they kind of targets, but their Disrupt ability will be much more effective against broken targets.
This raises a slightly'wacky' thought, should Void Spinners be +25 points for each additional unit , eg 275, 300, 325? Alternatively, should there be a limit on the number that can be bought in an army?
Personally I am not convinced, but thought I would raise the questions.
The main reason I am not sure about either is that they occupy a slot in the army, and that prevents using another formation. For this reason I cannot recollect ever seeing an army with three VS in it, even at Britcon where we use 4K armies.