Here's a thought, you mentioned the front toe is set to 0*, when a FWD car is under a load/accelerating, it will try to pull the front wheels toward the center-line of the car, rotating around the axis of whatever will allow it some give, typically the control arms or the knuckles. This is why they often have some positive toe, because it will tend to be pulled to 0* or slightly toe-in. With 400whp, and who knows how much torque, this problem would be even greater...
The thing that might exclude this as the culprit, is that you only seem to see it at high speed, that sounds like too much air getting under the hood causing lift, kinda like the Buick T-type turbos would do. The fix there, and possibly here too, is to prevent the air from going under the car (the air damn previously mentioned) and/or extracting that air, either via Trans-Am/GNX style fender vents or ??? I'm not sure if removing the drip pan would do the trick, it might help, but I'm not so sure it's in a low pressure area, being so close to the windshield, which creates a high pressure area (the reason cowl induction works!).
The LSD could be trying to shift torque from wheel to wheel, but that would always be there and more prevalent at lower speeds, or so I would think...
Given the suspension work you've done, and the fact you only see it at high speed, it sounds like you'll be doing some Aero work very soon!
Hope this helps!
Mike