I don't do a lot of off-road but I totally agree with your experience. It takes a lot less effort to pedal up a steep hill or over rough terrain when you're clipped in. It's shocking how many muscles you use just to stay on the bike with a platform pedal. Even on my road bike I found that cobblestones (I'm from Belgium ;p) are a breeze when clipped in, and a total b*tch when using platforms.

My shimano SPD pedals came with 4 plastic inserts that convert them to platform pedals. I currently run one insert on each side so I can use my mtb for a grocery run.