This is a high performance car. It is not optimized to drive 70MPH in 6th gear. If you don't like the sound in 6th gear, downshift to 5th or even 4th. The sound will change and you will have more fun besides.
In my experience, every aftermarket exhaust will be noisier than the stock. That's what they are designed to do.
If you want to cruise in silence at 70, this is not the car for you. I love the sound and feel of this car at any speed. No complaints here.