Been messing around with the idea of a pair of slavers flanking with a HWK in the centre popping ion all over the place. What are people's though? Could this list (or a variant of it) ever be any good...?

The issue with Dace in the middle is that you won't get to trigger his ability (or his cannon) very often; two large ships on either side provide a lot of base to shoot at while staying away from Dace's sphere of influence.

The other issue with this list is that it is abundantly easy to outmaneuver. While the Ion does provide some control, that mostly means that once the two lists crash into each other, yours will no longer be able to ionize due to bumping, while also being unable to turn around. Also, bear in mind, if you want to be using the ion cannons, you need to have them in the front arc, which can be easy to avoid.

Now, I'm not saying that it can't work, just that you have to keep those issues in mind. For example, strip off the EMP Device, and throw Tactical Jammer on both slavers. You can now run the ships in different formations, maybe keeping Dace closer to the action, but also having some interesting plays. Or, if you want the second one there as a support ship, you can toss the Inspiring Recruit on Palob with an Ion Cannon Turret, providing more debuff, allowing you to make those low-damage shots go through reliably, and also just generally messing with plans. And, then you have 7 additional points to work with, while also removing the problem of having Dace sandwiched between big ships (cutting into his bubble).

Now, that was just a suggestion, and again, I think your list can work. You just need to fly it with an eye for its flaws, do some testing, and find the flight pattern that works best for you.