Ion Drives
Everything I read about Ion Engines in the rules say that they're powerful, yet their performance as indicated by the rules says otherwise, same ADF as a Chemical Drive.

Theoretically speaking, since when is a Chemical Drive restricted to in-system only duty? If they accelerate at the same rate as Ion Drives, and since achieving jump velocity (200 hexes/turn according to the opening text on page 3 of the KH Campaign Book:

"12 million kph {1% of the speed of light}"

Go ahead and do the math...) is just a matter of acceleration, given enough fuel for a Chemical drive what real advantage is there in opting for Ion Drives?

Hence, Ion Drives in my campaign allow for an ADF of half (rounding up) that of similarly equipped Atomic Drive powered craft. They can also be mounted in the stern or on struts, seeing as they don't emit any radiation like atomic drives would.

(source - House Rules WIKI)