I installed the heavier spring, and I agree, it's an improvement. The stronger return-to-center makes it easier to find the 3-4 gate, most notable for me when doing a 5->4 downshift at speed (heel-and-toe, under hard braking, at the track). My thought while installing it was that 4x stiffer might be too stiff, but it really isn't.
Just a couple notes for others doing this upgrade:
The installation directions are good, but when bolting it back together, I found it easier to insert the left side white plastic washer first (before you insert the plastic washer on the right). The left washer space is harder to access, so it's easier to do the harder one first. Also, to get everything lined up, I went back and forth from side to side with the shoulder bolt. First I inserted it from the left side, and shifted the washer around until the bolt passed through the washer and into the tube, then I pulled out the bolt and did the same thing entering from the right side. Rinse and repeat. After the second or third time I was able to get the bolt all the way through to the opposite side (while entering from the right side).
Secondly (but earlier in the installation process), be careful not to struggle too hard while seating the CG mechanism Heim joint onto the gear lever pin. There are two round internal push rings that hold the Heim joint in the CG mechanism, you can see one of them in the photo above (it has 8 outer tabs). I didn't have things lined up quite right, I pushed a little too hard, and the right hand retaining ring popped out into my hand. These aren't reusable so I bought a bag of replacement rings from McMaster-Carr, now I have 99 spares if anyone else needs one (a bag of 100 cost me about $10). No doubt Shinoo would have sent me one if I asked, but I like having spares. Bottom line, unlike me, make sure you get the CG mechanism well situated before you try to get the CG arm seated onto the gear lever pin.
'08 Arctic Silver Exige S240