"Sometimes when I ride the geared bike on a tough hill I have cleared on SS I still can't believe it."
Absolutely. This happens to me all the time now. I'll go out on the geared rig on a trail I also ss on, and feel like I'm barely exerting myself.
And even though you get worked a lot faster, I think the variety of movement- the body english - required for riding with one gear offroad also actually leads to less chance of repetitive stress aches and pains. I've been mindful lately of standing up more on the geared bike for example and mixing it up, instead of just sitting there spinning.