Prompted by the news of a Ryzen 3900 (without -X) in early October, somebody tried different package power targets on a 3900X and reported Cinebench 20 results along with at-the-wall power consumption 
in a German forum. The test system had a lower-end HD 7750 GPU, a 650 W titanium-rated PSU, and was of course supplied from 230 V. Firmware was AGESA 1.0.0.3 ABBA.
The user calculated CB-points-per-Watt for package power in his post. I am showing CB-points-per-Watt for at-the-wall power instead.
comment  | TDP (W)  | PPT (W)  | TDC (A)  | EDC (A)  | CPU package power (W)  | Cinebench 20 points  | relative performance  | system power (W)  | relative system power  | points/W of system power  | relative efficiency | 
stock  | 105  | 142  | 95  | 140  | 127.5  | 7360  | 1.00  | 189  | 1.00  | 39  | 1.00  | 
hard limit  | 105  | 105  | 95  | 140  | 107.6  | 7119  | 0.97  | 161  | 0.85  | 44  | 1.14  | 
eco mode  | 65  | 88  | 60  | 90  | 89.1  | 6942  | 0.94  | 137  | 0.72  | 51  | 1.30  | 
hard limit  | 65  | 65  | 60  | 90  | 66.6  | 6426  | 0.87  | 107  | 0.57  | 60  | 1.54  | 
sweetspot  |   | 58  | 60  | 90  | 59.4  | 6125  | 0.83  | 98  | 0.52  | 62  | 1.60  | 
 
The performance degradation at lower power targets is rather moderate. Consequently, the lowest PPT tested is the optimum for performane-per-Watt not only when looking solely at CPU power consumption, but also when taking all of the platform power consumption overhead into account. I wonder how representative this image rendering workload may be for other computing scenarios.