Complications of CFRD (poorly controlled):
Infections due to decreased white blood cell count phagocystosis.

Phagocystosis is the ability to encourage white blood cells to destroy bacteria.

Increased viscosity of mucus secretions with hyperglycemia and dehydration.

Dehydration is caused by the body trying to flush the excess blood glucose out in the urine.

Increased protein catabolism with CF [...]

How might early diabetes cause CF clinical decline? It is currently unknown, but here are some theories:

Insulin Deficiency

Insulin is an anabolic hormone which promotes conservation of body protein, fat and carbohydrate stores.

Malnutrition and protein catabolism are clearly associated with death in CF.

Weight loss and decline in pulmonary function began 4-6 years before the onset of diabetes.
After two years of insulin therapy,weight returned to levels seen six years earlier and the decline in pulmonary function stabilized.
This suggests a cause and effect relationship between clinical decline and the pre-diabetic state.
Lanng, Diabetic mellitus in Cystic Fibrosis: effect of [...]

Interesting tidbits about CFRD:
Diabetic ketoacidosis (DKA) is rare. (I would love some more info on this).
Pancreatic insufficiency associated with CFRD.
A1c levels may underestimate abnormal glucose metabolism. (would love more info on this too). So far I have only found that there is an increased red cell turnover, causing a “diluted” A1c [...]