Can it be estimated when it would actually be beneficial to increase drag in tailwind?
A tailwind reduces your relative airspeed and therefor the drag you generate. You can go faster relative to the ground  with the same power output. The only time increasing your profile might be beneficial is if the wind is traveling faster than you are, relative to the ground .... Unless you cycle slowly or the wind is very fast, it's usually not the case. This is cycling, so aerodynamic is mostly about reducing drag, unlike sailing where maximising lift is important.