First off this is actually the incidence angle rather than the angle of attack. Or more accuratley the angle between the wing and the stabilizer is what's important and that is known as the decalage angle.

Secondly you're right. It is a lot but I've seen that sort of angle on many WW1 drawings. But most of our models tend to fly around with the Center of Gravity farther back than on full size aircraft. And to allow for that it's common to set up the models with less decalage than for full size.

Here's a good article about some of this. http://www.fatlion.com/sailplanes/decalage.html

The bottom line is that if you want to use the model in world scale competition then leave it alone. If it's for Sunday flying then you could do worse than to change it to the 3 degrees you're suggesting and compensate for any problems with a touch of elevator trim later. With 5 degrees you would be much more likely to need fairly noticable down trim for a nice flying model.