This might be the answer to my increased data usage. I have never, ever used Siri, and always had it turned off. However, per your post, I just checked Settings and Siri was turned ON. If somehow the iOS8 install turned it on and it's using data in the background that I'm unaware of, then that would explain things. Anyway, I turned it OFF and will see what happens next billing period.Interesting....I am sitting at home and my phone is connected to wifi. I reset the statistics under System Services in the Cellular settings. In 15 minutes "Siri" used 3.2KB of data. Now why is that? I am not using Siri and I am connected to wifi. To be honest, I have not been very impressed with Siri anyway.
Edit: I did some googling and didn't realize, but apparently Siri regularly connects with Apple servers to sync content, making it more likely to use data.