Great question. I believe the following article addresses your concerns:
Your iOS device initially uses a compact voice for both Siri and VoiceOver. After you have configured a Wi-Fi network and have the device connected to a power source, iOS will automatically download and install a more natural-sounding voice.
1. Go to Settings > General > Siri > Language.
2. Select the Language and Voice Gender* you want to use.
Note: If you change the language or gender for Siri, the new enhanced-quality voice will need to download.
I'm sorry if I'm missing something. Not sure if this is the problem the OP is having but I'm having a problem understanding Siri when she reads me texts. She uses No pause between words. Completely to fast. I went to voice control and put the slider on slow down but it doesn't slow down Siri when she reads me my text. I have to pull my phone out the holster, which is a pain with dirty hands, and read my texts. Was doing fine on iOS6.
Doesn't matter which headsets I use. Dr dre beat tours or apple ear pods