There is no mention to Triple Castellan void shields in ITS - although I think that's so due to the fact it was launched before Battlefleet Koronus (and the Grand Cruisers shields)?

So, in my opinion we could get the same bonus to a Grand Cruiser. (VS 3, tech use test to boost to 6)

But just for SP2?

Edit:

Space 3, Power 9, SP 2
Counts as 3 Void Shields. Once per Round, Enginseer Prime may make a -10 Tech-Use to double shield count for a single turn. Doesn't take extended action.

Any liabilities here? You have a very gifted AdMech, and they can, round after round, crank your void shields to 6? Does something go bad if he/she fails? Does it overstrain the mechanism? It just seems weird that nothing goes wrong, and going right, you laugh in a Battleship's face, or they'd mount theirs, and have 8 Void Shields

A sufficiently powerful TK Psyker can (and will) just hang around and reinforce Void Shields constantly to make your ship ridiculously tough for a single turn as well.

It would be very, very powerful, though you seem to be less concerned about that for your players. Remember that the boost can only be activated once per round, and only lasts for a single opponent's turn . Thus if you're up against two Cruisers at the start of an opponent's turn give your Explorator a chance to boost the shield array before they know how badly they're hit. Any bonus void shields are then "lost" at the next opponent's turn.

Sry, not taking into account the breakdown of time in starship combat, I guess.