Graynomad:
Actually if you look at the data sheet it shows a peak Vo of just over 3v at 3cm. And the other graph shows a Vo of between 2v7 and just under 3v depending on the reflectance.So what you are seeing is about right, and it looks like you don't need the divider.
Rob
But add a current-limiting resistor to prevent damage - that datasheet doesn't appear to give the output impedance or drive current level.
[And go back to the shop and tell them they were wrong 
]