Merg:
I use a heavy aluminum 055 Manfrotto tripod and the new heavy duty 3-way head 329RC4 with my SL66. I haven't had any issues and don't feel the need to get into a heavier rig. (longest lens 150 at the moment)
Now, onto another topic. The SL66 has issues with many tripod heads and quick releases: the tilt motion hits either the head or the plate and you cannot achieve the full 8 degrees of lower tilt. You can get around this if you have the Rollei quick release but it is tall, narrow and has the potential to greatly amplify vibrations IMO, not to mention it is expensive and not so easy to find.
If you are interested, I am looking into machining a small number of aluminum adapter plates to go between the camera and the quick release. Stay tuned for more.