I realized that BS to OMC DC PD throghput is measured today in klog #13476.
3 W at IMC output means 3 W * 10.35 % * 50 % * 50 % = 77.6 mW at BS output to SRs.
So, BS to OMC DC PD throughput is 3.3 mW / 77.6 mW = 4.3 %.

The updated theoretical quantum noise using this value is attached (see red dotted line).
Green curve is the measured shot noise in klog #12772, scaled with current input power and power at OMC DC PD.
They are consistent within 10 %.