[Drummer]

Solucionad0 gracias a la ayuda de Volvo:

for(int A = 0; A < HorzScrollBox1->Content->ChildrenCount; A++)
{
  static_cast<TGridLayout*>(HorzScrollBox1->Content->Children->Items[A])->Width = HorzScrollBox1->Width;
}
HorzScrollBox1->ViewportPosition = PointF(LastPanel * HorzScrollBox1->Width, 0.0);