As an inspector, I see a lot of things done that are a bit..different.
I saw one that had taken a furnace designed to be mounted
horizontally, under a floor in the crawl space.
Person had mounted it in the basement above his existing unit,
and the exhaust from the first was directed to the fire box of the
one above. That one sent its output to a couple of bedrooms
rather than having 'dueling fans'.
Indeed, that second one recovered considerable heat from the first
one, and the exhaust from it was still warm enough to maintain
good venting.
When I saw it, I was serving as a Health Dept. inspector on Haz-Mat
duty, and my carbon monoxide detector was routinely verified to be
in excellent function.
Rather than modifying the existing unit, you might consider
alternatives.

On Sep 12, 8:01*am, "JohnR66" wrote:
I heard that adjusting the gas valve on my standard natural gas furnace will
make it more efficient.
By partially closing the gas valve reduces the flame. Although the furnace
would run longer, the flue temp is reduced meaning lower heat loss and
improvement in efficiency.

An analogy is like driving a car the same distance once fast and once slow.
When driving slower, it obviously will take longer, the engine is on longer,
yet fuel usage is less.

I have no way to verify if this is BS or not.