I use oval rings because my right knee is most compromised coming over the top of the stroke and they anecdotally relieve pressure and reduce the constant inflammation. No, they do not help me directly produce more power, but allow me to train harder, and eventually generate more power through better training.
To add some more clarity on the over estimation, when the chain speed is constant, the angular speed of the cranks is fluctuating with oval rings. They are designed to be slowest in the strongest part of the pedal stoke as the radius increases. Power meters that use average angular speed overestimate the angular speed the greatest when the torque is the highest, but underestimate the speed when the torque is the lowest. The overestimate is greater because of the torque distribution causing the offset. If you spin a perfect circle there would be minimal error. If 100% of your torque is concentrated at the highest point, the error would be roughly half the oval percent. If you distributed 50% at the highest point and 25% at plus and minus 45 degrees, the error would roughly be one quarter of the oval percent. Absolute Black lists as 10-11% oval for reference.
The error is fairly predictable and most likely within the range of accuracy of most power meters. As long as you have consistent readings, I wouldn’t be concerned with the error impacting your training.