It all depends on how sacrosanct your stabilised approach criteria is. If you need to be stabilised at 500ft AGL on final, you must turn base abeam that point (ie 1.6nm/2900m/40sec at 140GS). It is impossible to remain within circling visibility criteria from the threshold (we need to keep the runway environment in sight;not withstanding metallic obstacles to vision!) while still complying with the turn point.
Flyboy Mike's outfit realises that by ruling:
ATP CIRC APPCH - VMC ONLY
CL-65 ERJ-170 ERJ-190 CIRC APPCH - VMC ONLY
circling in VMC only.
and Rat 5's FOI was also onto it:
The local XAA FOI did a calculation regarding the radius of a continuos turn from downwind to finals. He concluded that 2400m was not enough to keep the runway in sight at the spacing required to make a rate 1 turn onto finals. He enforced 3600m (CAT D) on the airline. We disagreed by performing in the sim and found a downwind spacing of 2400m to be sufficient for the turn. No success. His opinion was that 2400m vis at 500' agl was not healthy and that was it.
VMC flight at low level can be safely accomplished within a 4.2nm circling area so you do retain MDA protection; not sure about in a TERPS area...
As for altitudes, it is obvious that 3" per hundred doesn't work: If you're circling at 500ft, you're not going to turn at 15" and if you have any hope of getting wings level by 500ft on final on a 3° slope.