Per :
can I assume that the same energy and
exertion that I put into pedaling the mt. bike will be at the very
least 2 or more times effective when pedaling the bike with the very
thin tires.

No way.

On a straight, flat, paved surface I estimate 15-20% advantage for 1.25" slicks
at 90 psi over 2.5" moderate tread Mutano Raptors at 35 psi.... and judging from
other threads, my estimate is generous.

Not only that, but I have no way of telling how much diff is from fat/soft vs
skinney/hard and how much is from sick vs the "AeroDyne" effect of the tread on
the fat boys.

Also, the surface is an important variant. On pavement, no question
hard/skinney rolls better. When you get to rough ground, soft ground, grassy
surfaces, gravelly surfaces there's a crossover point. I can think of plenty
situations where I've ridden both tires and big/soft rolls about ten times
easier than skinney/hard.
--
PeteCresswell