ATtiny85 power consumption double expected
You say that according to Figure 22-7 in the datasheet it should only draw 300µA, but that graph shows the current draw for operation without clock division. A crystal oscillator running at 16MHz is bound to draw more current than one running at 2MHz, and the 3 stage divider will add a bit more. The question is - how much more?

The datasheet also suggests that idle current can be reduced by dividing the clock down, but again it doesn't say how much it will be reduced. Extrapolating the 3.3V line suggests that it would normally draw about 1.5mA at 16.4MHz, and 850µA is a significant reduction - but should it be less? 

If you can't use a lower frequency crystal on the boards you have coming then there may be nothing you can do. However, while you have the circuit on a breadboard you could at least try a 2MHz crystal, to see if that really is the problem.