I would think that you could adjust the temperature at which the bimetalic spiral locks up the clutch by moving the attachment point of the center of the spiral.
Twist the center attachment point forward, or add a spacer that creates more tension in the spiral to lock up at a lower temperaure.
To get the fan blades to rotate closer to engine shaft rpm, replace the silicone fluid inside the clutch with a fluid of higher viscosity.
Silicone is used because it is fire resistant.
Perhaps high viscosity silicone grease could be mixed into the original silicone fluid fill until you got the lesser amount of slip you desire.
http://www.amazon.com/Jet-Lube-Silicone-...=silicone+fluidA clutch manufacturer like Hayden might be able to supply you with replacement silicone fluid of various viscosities, rather than a complete new clutch assembly.
http://www.haydenauto.com/Hayden%20Custo...rm/Content.aspxhttp://www.haydenauto.com/online%20catalogs-ecatalog/content.aspxA solid 9 blade fan with the right spacer might be the simplest first thing to try.
