So what I've caught just from playing around with it, is that it is doing something, in that the bar is constantly flickering.
What seems to be happening is that the speed is being set higher, then reset to its base. As for why... Could it be because the modifier is the difference, so once it's set high, the difference becomes 0 and it effectively unapplies itself?
Edit: Yup, that's definitely it. Not sure how I can fix it to take account of if the core has speed boosts, but ignoring that inconvenient fact, I now have it working.