pakeha-boy,
Deployment of the RAT would indeed increase the form / profile / parasite drag of the aircraft, and cause the Vmd (Green Dot Speed) to be further reduced, but......
Deployment of the RAT is not normally an option considered following engine failure. Even for a 2 engined aircraft down to one operative engine, the remaining supply of Hydraulic, Electrical, and Pneumatic services should be more than adequately provided for by the remaining engine (not that the RAT would help the pneumatics).
What IS routinely considered following engine failure on a 2 engined aircraft is in-flight start of the APU, which will provide complete back-up of Hydraulic, Electrical, and Pneumatic services. Drag from APU inlet doors etc. will increase overall drag, but this additional drag must be balanced against any small thrust advantage that the APU might provide, depending upon the aircraft type.
Thanks for the nice PM!
Regards,
Old Smokey