The friction plat #7 that is in there now has a larger ID than the other plates. #4 , 5, and 6 are in that larger ID. The #7 plate because of the bigger ID has a friction band that is half the width of the other plates, the new plate will be identical to all of the other plates this will give you 50% more grip at that location which seems to make all the difference in the world.
Once you install this new plate and heavy spring you will notice slightly more pull need to pull in the clutch from the heavier spring not bad, just more than there was. The other thing that you will notice is that the friction zone will be narrower and the clutch will not engage until just before the lever is all of the way out. This is just how it work out.
For checking your friction discs for wear.
A new disk is 3mm (.118 inch) thick, the wear limit is 2.8mm (.110 thick). That aint much. only 0.1mm (.004 inch) of material on each side between new and worn out. I would not replace the disks unless they are worn more than 50%, 2.9mm (.114 inch).