Formerly known as Augmented Engines, this hull mod was right there with Integrated Targeting Core for "most valuable hull mod" contention. It increased Burn speed (a huge deal) and it also increased in-combat speed. It was a "must have" for most ships and contributed to the high-speed combat of pre-0.8a.
For the latest patch, it was given a new name, only increases burn speed, has a sensor penalty (both ways) and is locked behind difficult-to-salvage Research Stations or (typically) Tri-Tach Cooperative Commission Military markets. Occasionally, one may drop for you but it's incredibly rare.
With the addition of Sustained Burn, I believe the pendulum swung too far the other direction: it's practically worthless now. Even with proposed changes to S-Burn (i.e. AI fleets will use it), +1 burn just isn't worth 5/8/15/20 OP and 50% less Sensors/+50% greater sensor profile. I don't know if I'd mount if or 0 OP cost yet its being treated like a god-tier hull mod.
I'm of the opinion that it should remain a highly-prized/rare hull mod so I'm ok with it being a bit on the powerful side. The suggestions that follow are based on that premise:
1.) -35% Fuel Use (no penalties). OP Cost reduced to 2/5/10/15. Rather than making ships go faster, it makes them more fuel efficient. Useful for capitals that are fuel hogs but you can squeeze it in on a lot of ships to really reduce your fuel consumption. Good for exploration. Stacks with Navigation 2 bonus. Since this is a per-ship mod, you can pick and choose where its appropriate.
2.) +10% Top Speed, +25 0-flux speed boost. Negative effect on Coordinated Maneuvers bonus (-1/2/3/4%). If you have Coordinated Maneuvers, ships with ADF will not contribute to the bonus (unless you stack Nav Relay on top of it) and if you don't have it, ships without ADF will be slowed down by the debuff. Numbers might need to be tweaked but the point is putting this on a ship makes that ship faster at the expense of slowing everything else down a little bit. Doesn't have the range penalty of Unstable Injector but is overall slower.
3.) +3 Burn, +25 0-flux speed boost. Sensor penalties intact. Mostly for capitals, ADF makes Level 3 Navigation a bit less of a must-have because you slap ADF on the one or two offending ships that are dragging your fleet down. In-combat, it gives the slowest ships a reasonable speed boost before the shooting starts. Also makes Level 3 Helmsmanship a bit more attractive, even for already fast ships.
Other ideas would be welcome.