I've got two car seats in the back, even have the rear facing seat on the driver side. At 6ft even I am about at the limit of where I'd like to have the seat and still be comfortable. I could see if I were a few inches taller it wouldn't be very comfortable.
I have the Corsa Dual after watching a few of his videos. I really wanted the Borla but the price was just too extreme for the axle back. I have zero regrets getting the Corsa cat-back. Sounds amazing and the price is right.
If you want big gains it really will be more about driving habits. Earlier in the year I was under 17mpg, I started to change how heavy my foot was on the gas and increased my coasting distances coming to stops. Since then I'm steadily over 20mpg per tank of gas. I've also found Eco mode does...