"...the throttle position switch...thought that it's primary function was to determine a closed throttle position so the idle motor kicks in..."

I think that's its only purpose.

"...from a dead start I have no Get-Up-&-Go. My wagon is just plain lethargic..."

I helped fix a friend's car (non turbo '87 740) with precisely this symptom. The acceleration from park was dangerously slow—for example, an on-ramp to the expressway was life threatening until about 35 mph.

But the car ran OK at highway speed—not a rocket ship, but not dangerous.

It turned out to be a sheared alignment pin between the cam and cam sprocket. Though the belt and sprockets were all properly timed, the cam had lost timing relative to its sprocket.