I fancy going on to clipless, recommendations anyone?

Hold on a cotton picking minutes, stiffer shoes does not necessary mean it'll be uncomfortable as long it flex a little bit, walking boots are similar as they tend to be a lots stiffer than conventional shoes (althought they're really uncomfortable when trying to break them in in the first place), even if the MTB shoes does not have carbon soles.

As for sacrificing power/efficiency, it doesn't really matter that much when you're riding your bike on a day to day basis, it's only a drawback when you're doing TT or overtaking nodders on the Spice Route, it's still a lots stiffer and effective than clips.

Ed, I also sold hiking/walking boots for nearly 4 years when I was younger along with all that technical clothing I was rambling on about earlier this week.
You are talking out of your arse mate, walking boot construction is so far removed from cycling shoe construction its borderline comedic.

Walking boot construction key points:

Moderate reverse camber sole to promote an efficient roll of the foot, from the heel to the ball of your foot, when you climb/walk/hike...whatever.

Slightly raised heel profile, dependent on boot design, but again... in aid of promoting the above.

Walking boots when stiffened, for instance Meindls top end boots (which use Vibram soles), use a more exaggerated camber to take into account the added stiffness, which in turn provides better protection for your ankle and reduces fatigue when hiking for long periods of time over rough terrain, as this stabilizes the foot better.

In the stiffer boots especially, the aim is to eliminate side to side flex, and backwards flex to give you more leverage/support, but to still allow the boot to flex forwards enough when hiking up steep ascents.

In summary, what you require from a hiking boot, compared to what you require from a cycling shoe are completely different.

Try and walk over any kind of long distance in a cycling shoe even if its MTB specific is not gonna be good for your overall knee/foot health, because of the lowered heel cup, flattened sole (in comparison to a hiking boot anyway), and the lack of any kind of flex/camber in the sole (in the stiffer high performance models anyway) amongst many other things.
Any decent cycling shoe should be stiffened to optimize power transfer, this makes it less comfortable and less healthy for your feet knees to be walking in over long distances, cycling shoe manufacturers don't carry out the necessary compensation for stiffening in specific areas that a hiking boot manufacturer would carry out for instance, as they are not intended for that purpose.
I don't know a lot about cycling shoes but I know enough about shoe/boot construction to know what counts where.