What's the health history of her line? I'd be less concerned with possible aggression (studies on that show overall increase in anxiety based behaviors including aggression, which coincides with reports from humans who have undergone similar surgeries) then the possibility of diabilitating injuries and life threatening cancers.
PLOS ONE: Neutering Dogs: Effects on Joint Disorders and Cancers in Golden Retrievers
Will Spaying/Neutering Increase Your Dog's Risk of Cancer?
Spayed dogs are more significantly more likely to suffer from CCL tears. (Sex hormones are responsible of muscle mass, fat to muscle ratios, and bone density). If your dog is going to be highly active this could be a deal breaker on its own.
Spayed females are 4.3 times more likely to develop lymphoma.
Spayed females are 9 times more likely to develop hemangiosarcoma. (If you aren't familiar with how devestating hemangio is with GSDs there is a sticky either over in preparing to say goodbye or the health section on this forum. Devestating. Takes them fast. Not really treatable. Common in our breed)
Spayed females are 3.5 times more likely to develop mast cell tumors.
All this in addition to the increase of possible unwanted behavioral changes.
I did find the rottweiler study to be interesting. Females spayed after the age of 6 were 4.6 times more likely to reach the age of 13 than females spayed earlier. Hormones are important for overall health and longevity.
Personally I wouldn't spay. Do some research on pyometra (pyo and mammary cancer are the reasons often cited as why to spay- the boat has kinda sailed on you preventing mammary cancer though..) and decide where you want to place your bets.
You may also consider an ovary sparing spay. No heats, no chance of pregnancy, no pyo - but your girl reaps the hormonal benefits of having her ovaries intact.
Personally I would not spay. I'd be sure to be on the look out for pyo as the dog ages. But I figure nature has designed dogs to be perfect biological systems. I'm not going to tinker with that. Endocrine deficiencies have been widely studied across dozens of species and pretty much the results are the same - hormones are really important for many body systems beyond reproduction.