It depends on the table and the shelf.
As Dan_Ed and Goldeneraguy advised, the Inverse Risers (firmly affixed to the underside of the table or shelf) are essential for the security of the rig.
The Standard Risers may or may not improve sound, I expect it might be table and shelf specific. I use them but in truth I'm too lazy to A/B without them. Moving 80 lbs. of table is just so much fun. Besides, they're threaded and provide a handy way to level the table.
High mass unsuspended tables like Dan's Galibier Gavia or my Teres 320 seem to benefit in that an already low sound floor is further lowered. More low level detail, greater clarity between notes, richer natural harmonics and longer natural decays, etc. Like Dan we experience a slight diminishment in bass slam/macrodynamics vs. using spikes, but for our tastes and system the benefits outweighed the costs. Definitely a trade off and a matter of taste.
In theory, we should want to keep isolation devices (which allow movement, by definition) as far from a table/arm/cartridge as possible. It's the SP's movement that accounts for that tiny loss of slam, as well as the reduced noise floor. As Brf suggested, an appropriately tuned resonance-controlling shelf directly beneath the table might allow the SP's to be placed one level lower. That should reduce the cost in bass and dynamic slam. The more mass suspended on the SP's, the lower the suspended system's resonance frequency, presumably taking it further away from audible frequencies and reducing the negative effects on transients.
In practice, much would depend on the characteristics of the intermediate shelf.
