With stock components, available pressure disk travel is 4mm and the release mechanism travel is only 1.8mm.  That leaves 2.2mm grace (.087”).  Keep in mind that we are looking at the stock clutch.
 
Adding extra plates not only increases the friction area, it also decreases the installed height of the springs.  That decrease in height increases spring preload.  What’s not to like?  More friction area + more spring force = higher torque limit.  The clutch slip is a thing of the past.   Seems like a good idea, right?

Ooooops!
 
What about travel?  Increasing the stack height of the plates eats up all the travel.  You might end up with an inoperable clutch and/or a broken release cam and/or broken something else.  You just can’t throw in a pair of extra plates.

If you start adding plates, that grace evaporates in short order.  The fiber plates are .115” thick and the steels are .062”.  Add one of each (a pair) and your .087” grace just turned into about .090” interference.

How about thinning down the plates?  There are thirteen plates.  If you remove .010” from each plate you would reduce the stack height by .130”.  That might fit, but it would be a lot of precise work.  Do you happen to have a surface grinder with a magnetic table?  That would work on the steel plates, but the fibers are a different story.
 
If the stock clutch has .157” travel, and you add a pair of plates (.115” + .062” = .177”) you reduce the travel to minus .020”.  It’s an interference fit.  Break out the sanders and grinders and lappers and whatnot, and lob .130” off the stack, and you could possibly restore travel to .110”.  That would give you a .040” margin over the release mechanism travel.  It would also reduce the spring installed height by around .047”, which would increase the spring preload.  I don’t think the additional spring preload would be a showstopper, but ya gotta keep that release cam in mind.

What about the oil slots in the friction material?  The friction material is right at .015” thick.  You could take .005” off each side.  That would make the oil slots shallower.  I think those are there to provide a path for the oil to be squeezed out when the clutch is engaged.  Would shallower slots cause problems with friction?

You can see from this photo that the material is very thin.  The total thickness is .115”.  The metal portion is .085”.  That leaves .030” total friction material, or .015” on each side.  Are you willing to sacrifice 33% of your available friction material?

How about just adding an additional steel plate?  That would increase the spring preload by 1.5mm (.06”).  It would decrease pressure disk travel by the same amount.  You would end up with total travel around .100”.  It looks like a reasonable option to patch up a worn clutch or even beef up a clutch that’s just barely at it’s limit.  But again, the release cam will be subjected to more stress.  It can probably handle the additional 24 to 28 lbs of total spring load, but there are no guarantees.  For that matter, there are no guarantees on the stock setup either.  We really don’t know for sure why the release cam fails.

I think the extra steel plate option might be a good approach for a worn-out clutch.  For instance, if the hub flange is worn down a millimeter or so, adding an additional steel plate would bring the stack height back close to the original height.  You could get a few more miles out of the old horse.  If you add an extra steel plate, it must go on the pressure disk side of the stack.  It won’t fit against the hub flange.

If you want to install an extra steel plate to compensate for wear or increase spring preload, install the extra plate on the pressure disk side of the stack, like this.  But remember, it will probably increase spring preload and stress on the release cam.

If the pressure disk disengages from the hub, the splines can hang up.  So, if you choose to mess around with adding plates, keep this engagement in mind.  Make sure the hub protrudes out of the plates at least 2.5mm (.100”) to maintain spline engagement when the clutch lever is pulled.

Anything you do that changes the clutch plate stack height will alter the release mechanism setup.  Replace a worn clutch, you must check the release mechanism setup.  Add more plates, you will most likely have to install a longer pushrod.  It comes in three lengths, 44.5mm, 45.5mm, and 46.5mm.  I suspect you could fabricate one from a ¼” drill blank but the end radii would require good attention to detail.  So far, the three available sizes have served my needs.

I was able to tighten things up by installing a .008” shim between the shaft and the cam.  The shim takes up a lot of the excess clearance and forces the cam to bear all the way across the flat.  I don’t know if it will prevent the cam from failing, but I don’t think it will hurt.

The ball socket in the release cam was galled.  I replaced the cam.  New cam on top, galled cam on bottom.