Problem: 5-speed manual shifter is easier to pull left (1st and 2nd) then right (5th and reverse)

Explainer: Noticed this after IRP shifter install. Seems like it is a known finding according to their support tech. Tried replacing the factory spring in case it was worn to find no difference. old spring in my 240k miata was the same height and felt the same in terms of springiness. Feels very light to go over to 1st and 2nd, feels just about right shifting into 5th and reverse. Note this was not as noticeable with a factory height shifter, but with increase leverage with a taller shifter, amplifies the feeling and generates a discrepancy. As you don't get good feedback when you are centered, I end up applying more than needed force to (center) and end up trying to get between gears. Especially egregious shifting up into third under cornering. You can get it reliably by just palming up on the shifter, but hard to relax enough to do while cornering. Many a crunch on the last autocross day which required shifting into 3rd while navigating stepped boxes. Also have noticed guest drivers do the same, finding difficulty with third until you inform them of just palming. Would be nice to not have to give that warning.

Info:
The factory replacement spring for the centering spring for first and second sits on the right side of the earlier 5-speed transmission

Seen on the left side of the diagram with the square plate, also on the right side both behind the ball and to the side
Part M501-17-416 is the spring in question, used also on the 5th gear and reverse side, but a much stronger feeling, perhaps due to the double effect as it is also in the reverse lockout, or the angle is much steeper?

Factory Spring dimensions:
OD: 6.5mm
ID: 5mm
Loose height: 28mm
Compressed height: 12mm
Spring constant: unknown

Proposition:
Replacement spring with either increased spring free height to preload, increased spring rate, or both to increase the force supplied to match the centering force of 5 and reverse side.

Steps so far:
6.5mm is not a common OD, if I remember, there was room to go slightly bigger, but have ordered both larger and smaller OD springs to trial
ID was not a restriction at least on the side in question, so smaller spring ID should be OK, meaning thicker coils and higher spring rates should be achievable
Have ordered 10 candidate springs to try. Will keep progress here for anyone trying to copy.