It seems like everyone is in agreement about mimic engines being a problem, the core issue of which is the way they offer almost zero interactivity from the other player (i.e. "you just sit there and do nothing until I alpha strike you").
I like the ideas going around of just making it a Ld test with a penalty (either take the test on 3D6 like horizon suggested, or with reduced Ld like Seahawk suggested). My thought was something along these lines:
When a Dark Eldar vessel is deployed or moves onto the table, mark it as mimicking (using a token or other note). Whenever an enemy vessel wishes to fire at a mimicking ship (including torpedoes and nova cannon, if the initial path/placement of the attack will not overlap a revealed target), it must first take a leadership test [with a penalty TBD]. If the test is passed, the attack is resolved normally. Otherwise, the enemy vessel may not fire its weapons or launch any ordnance except attack craft.
Mimicking vessels are treated as friendly vessels by any ordnance with automatic targeting (such as mines, minefields, seeking torpedoes, etc.).
A Dark Eldar vessel is revealed immediately after it shoots, launches ordnance, moves within 30cm of an enemy ship (at any point during either player's turn), or defends against enemy attack craft or boarding torpedoes.
A Dark Eldar vessel may re-establish its mimicking state by feinting. To do this, the vessel must end its move more than 30cm from any enemy ship and attempt to disengage. If the leadership test is passed on a score of X or less [TBD], you may mark the vessel as mimicking once again, instead of removing it from the board.
Lastly, the rules for mimic engines replace the normal rules for target priority when firing at Dark Eldar ships. Note that this means an enemy may always choose to shoot at any revealed Dark Eldar vessel without restriction, regardless of whether or not it is the closest target.
Necrons & Tyranids still ignore the rules for mimic engines, and will use the normal rules for target priority.