"I had concluded the same as Donald."

Well, on reflection I think we're both wrong.

The way it works with Atmel 6/12 clock switchable devices (which bear a remarkable similarity to Philips devices!) is this:

The core always runs at 6 clocks per instruction cycle. If X2 ('6 clock') mode is selected, the full oscillator frequency is fed to the core. If X1 ('12 clock') mode is selected the oscillator frequency is divided by two before being fed to the core.

This means that a given device running with a 10MHz osc. in X2 mode and the same device running with a 20MHz osc. in X1 mode will both have their cores clocked at 10MHz. The only power savings you are likely to see are from the (actual power consumption of the) slower crystal/oscillator and any individual peripheral clocks you are able to run in '12 clock' mode (there are individual dividers for each peripheral clock).