Well mine doesnt slip really at all... I was expecting some, but its really bad. If I was to do anything different while still using this clutch it would be to use a heavier flywheel.
Wow, deja vu. Reading between the lines, I'm guessing the car can't be launched unless you rev & dump? Can't let it out smoothly from a "normal" start without the car stalling? Starting off from an incline is nearly impossible without stalling?
I've been saying these things about my TRD Twin Disc for the last three years, and along with needing a heavier flywheel, perhaps the discs need refacing with something a bit more slippable, like organic or kevlar (as suggested by TurboAndrew). These multi-disc clutches with lightweight flywheels just aren't much fun to drive on the street.
Perhaps the best solution for the street is a standard heavy steel flywheel, and the largest diameter, single disc friction surface possible to hold the torque (a la Zuffen's clutch, also referred to as a "granny clutch" by Justen, hehehe). That or perhaps a carbon/carbon setup, which is $$$$.
One suggestion I can offer for these QM setups is to "tune" the hydraulic ratio between the master & slave cylinders. By
increasing the diameter of the slave piston, and/or
decreasing the diameter of the master, you can achieve
less stroke of the slave for the same stroke of the master. In this way, the system can be made to "feel" more sensitive.
In other words, if your clutch bites really hard as soon as your foot is off the floor, maybe you're not using all the stroke of the master cylinder and/or you're overstroking the slave? I've always wondered about this when I hear about people "popping" the slave piston out of its cylinder on these hydraulic TOB setups.
I had my slave cylinder bored out 1/8" and it made a noticeable difference with my TRD clutch setup.
Obviously with a hydraulic throw out bearing, you probably can't adjust that side, so you'd have to work from the master cylinder side and sleeve that cylinder diameter down.
Here's a link for a gentleman who specialises in this type of work, and has already done three slave cylinders for me and at least two of my Supra buddies that I know about. His name is Joe Way, and his company is Sierra Specialties, Inc. A really nice guy who does very good work at reasonable prices:
http://www.brakecylinder.com
What a pain in the ass. We spun up a freemachining spacer on the back like you did and actually used a valve seat to spacer the bearing out more. So it was set up perfectly, and then ONLY 4th gear would slip! So weird all others were fine, remove gearbox and surface another 2mm off the spacer and no slipping. We cant explain it haha. 
