To my way of thinking that extra spring is adding unnatural friction to the throttle assy and would be undesirable regarding throttle action. To gain better pedal feel, it might pay you to deal with that issue at the pedal shaft or lever itself, so strain on the throttle assy is not affected.
My TP was similarly temperamental when I first acquired the car in 1993, but after I got copies of the literature and came to grips with the innards and their friction and lateral disc alignment issues in 1995, the Throttle assy has not been a problem since then, 16 years trouble free.
Scrupulous internal cleanliness and regular lube, are key.
The main problem is distortion of the rear spindle caused by internal friction and over-tensioned centre springs.
Lack of cleanliness also plays its part, as does the lateral misalignment of the discs in their tunnels - all mentioned in the pdf.
If you add tension to the centre as shown in these images that I collected over the years, then expect more problems than solutions. The rear spindle is simple not up to it.
others have plumped for centre pull as seen in the next image.
For the record I have no knowledge of whether it is good or bad, but at least the rear spindle is not compromised.
Sorry about image quality but they were cropped from images found on the web - but you get the idea.
