Antenna Design Necessary?
The problem with consumption is only marginally related to the antenna in this case. The only dependency is that with a better antenna you can decrease the error ratio, reducing the total amount of sent data. But if you want to reduce significantly the power, you have to play with the power management options of the transceiver, the chapter 5 of the manual.

Probably you have already read it and optimised the application, but if you want to sacrifice range and reduce power, the most obvious solution is to lower the transmit power, as described in section 5.5 (page 18).

The cantenna reduces only the power irradiated in other directions, but not the overall power irradiated; moreover, changing the antenna, yes, can affect the power consumed by varying the antenna impedance and lowering the power transferred, but you pay it with a loss in efficiency.

IMO the best thing to do is to optimise the radiation surface of the antenna and then reduce the transmit power to the working minimum.

Note that if you decrease the power too much, it may result in a high number of lost packets, and then in a higher consumption.