Been towing with omega's for quite a few years now .. firstly the 2.5 now the 3.2, both auto's, both brilliant towcars.
Additional gearbox cooling is NOT needed. Even in the mountains of the South of France with the temperature at 34 degrees, neither the gearbox or the engine overheated
An average of 20 when towing is good, you'll get a tad better if you a) don't use all that loverly power to accelerate hard (I know....its fun winning the traffic light GP with a 'van on the back as well .. but it costs !!) b) you sit at around 50-55 on the motorway .. not 60-65, makes a big difference to consumption. Don't be tempted to sit at 40-45 .. you'll probably not be in 4th. Also, cruise control will try and keep you at speed even up-hill, so the system will change down rather than slow down, its more economical to stay in top and lose 5-10 mph.
You have more than enough grunt to tow at above legal speeds anyway, just make sure you do regular oil changes and air filter changes, this will ensure ALL the horses are saddled up and waiting ..

You have one of the best towcars around .. relax and enjoy it ... you'll arive on-site cool, calm, and comfortable ..

HTH