Yeah, top speed with OD on is about 105-108,
OD-off allows you to go up to about 125-128.

The reason is because the engine is too underpowered at the lower RPM. If you downshift by turning OD off, you get just a little more HP in the higher RPM range.

Just look at any dyno graph and you'll see the car makes max HP at the higher RPM ranges. If you're not in the high RPM range, you're not making enough horsepower to overcome aerodynamic drag.

Another aspect to consider is the heat of the transmission. An automatic generates a lot of heat in the torque convertor. If you overheat the tranny, I think the A604 is capable of informing the ECU about it. The ECU might be able to cut back the power, although I've never actually seen this happen on my car.