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A600 is pretty much the lightest you can go without compromising functionality, it work like a road pedals as it weighted to allow clip in at an angle.
You can get those Ritchey double sided spd pedals if you're willing to spend a lots of money just to save weight (£100+ for around 230g)
IMHO, stick with the cheap Shimano SPD, pedals weight make sweet fuck all difference.
Edit; by cheap, I mean those M540 pedals that's only about 40g heavier than the top of the range XTR.
I saw some nice, lightweight spd pedals mentioned somewhere, I know it's vague, but anyone have any ideas what they would be? Not Shimano A600