The pressure in the scavenge circuit is very high when cold. But the key is that this section of the oil path with the trombone loop is blocked on the return side when cold. It's just a "stack" of oil that is waiting to move until the thermostat opens. So there is no flow and thus no pressure. The point I'm getting at, is this part of the oil system isn't under pressure until the oil at the thermostat is just above 180F
But for reference, here's an example of the pressure created by the pump elsewhere when cold. If you don't run a thermostat before the oil cooler, you risk damage to the cooler and other things when the oil is cold and thick
http://forums.pelicanparts.com/911-engine-rebuilding-forum/632726-911-3-0l-sc-oil-cooler-system.html#post6308846