I didn't know that's what you meant by "open space battle".  I thought you were being metaphorical, as in "do whatever you want."
Also, don't know if it will effect my ship cost, but tachyons interact very weakly with dark matter.  It only shows up at long range if you have a lot of dark matter, like a nebula or a dreadnaught.  Smaller ships can't be detected as far away while cloaked.