I believe it's the fact that when in LOC capture mode, the autopilot will only start to turn when the LOC needle starts to move, and yes, above 180knots the turn radius is too large and the aircraft will shoot through. Experienced pilots are aware of this, and use the following methods to avoid it:
Reduce speed to <185 kts.
Intercept angle of 30 degrees max. (Also, helping out the autopilot by easing it onto the localiser using HDG).
Better still, keep the autopilot in FMS mode, then the autopilot 'knows' when to start the turn to allow for the correct turn radius. Switch to ILS when the aircraft is established on the final approach track, or at least in the turn to intercept.
Having said all of that, the Q400 autopilot often just does a crap job regardless of how much you try to help it out....