Can a sun tunnel be installed with a 15 to 20 degree angle in it?
Whether or not they can be bent, and the degree to which they can be bent, depends on the manufacturer. 

The amount of light will be impacted, but the question is by how much. Light is lost the more reflections required to reach the diffuser. 

I would say you don't need to worry to much if you have one bend. Where it will get bad is if you have multiple bends and (in the case of flexible-reflector units) if the film is not drawn tight.