Out of curiosity, what are the specs of the crank that's on the FS MTB that you're trying? I know there are are other compact doubles with combinations different from the one I use on my roadbike (50-34), but I don't specifically know what they are.
For what it's worth, I personally would not like a compact set up with a wide cassette. As it is, I'm not overly fond of my current set up of a 50-34 compact and a 13-26 cassette because half of the time on the flats, I simply can't find a gear that makes my knees/lungs all that happy. I do have the climbing gears that I need most of the time, but the vast majority of my rides don't involve much climbing. Frankly, the only reason I'm running a compact to begin with is that my IT band wasn't happy with my triple. I know Jonathan has discussed Q factor with you and doesn't feel that you're as bad off as I was with a triple, but what do you/he think you'd otherwise gain by running a compact over a triple? Weight savings? Crisper shifting?
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher