Short answer is YES.

The steerer simply isn't designed for cantilever that far. Simply put, the leverage offered by the 5" lever arm effectively doubles the potential stresses on the tube.

That said, it might be OK depending on the rider and how he works the bar, for example when climbing. So it's up to the rider to make a judgement call.

FWIW the question in and of itself implies that the frame is too small. The OP should consider a taller frame, or one with a sloping top tube to raise the head enough stay with conventional stack height.