Hey Slakr,
The reason the mugen shoes and springs might not be working for you could because of the geometery of the clutch.
If you compare the stock CEN clutch nut to the mugen nut, the groove where the springs are retained are in very different positions. I tried running the mugen shoes and springs once with the stock CEN clutch nut, it seemed as if something was wrong with the clutch, the same slipping problem you are talking about.
If you are going to use the Mugen shoes, you might want to use the Mugen flywheel, clutch nut as well for proper clutch geometry. Might also want to ask Gene for his opinion as well. This is just strictly from my own obeservation.
For the buggy I run the blue CEN shoes with 1.0 springs and then for truggy blue shoes with the 1.1 springs.