1. A runabout. 2. A Romulan cloak. 3. The rifle from "Field Of Fire". 4. Scotty's transwarp beaming equation (fed into the runabout, and the rifle). And, bam, you're invincible. Enjoy.
Thanks to the influence of several powerful warp drive manufacturers, the security apparatus of Starfleet Command, and a late night visit by a Section 31 team to "explain things" to Mr. Scott, you can be sure that transwarp beaming equation will never again see the light of day.
Pretty sure the Borg would figure things out before you picked off all of their hundreds of ships. Also there's always that pesky ship who blunders into an anomaly or the slingshot effect and goes back in time to stop you.
I forget. Did they ever recover Khan's TW device or do the Klingons have a chance of finding it when they locate that missing patrol?
Scotty found it in the wreckage of the ship Khan used to attack Starfleet Headquarters; it was never on Qo'noS. We can probably hand-wave away the possibility of transwarp transporters replacing starships by supposing that the plans to build them were lost when Section 31's secret installation was destroyed. But, really, if such a technology is possible, you'd be foolish not to develop it. Think about it: Khan transported from Earth across several light years of interstellar space to a PRECISE location on the surface of Qo'noS using whatever was inside that box. THAT is efficient. And the sheer improbability of such precision suggests that the technology must be very, very reliable.
