When I bought my car (brand new, stock) the sticker said "18 City, 24 Highway".

I drive 75% highway and was getting 21-23MPG.

I have done so many mods besides just exhaust mods, but now I am getting 24-26mpg. A 500 mile trip to Buffalo NY netted me 27.9! - not bad for a 400+HP engine. When I upgrade my turbo the mileage will really increase. It will be harder to accidentally get the engine into boost. Friends have claimed 30+ on the highway with similar setups.

The main things helping this are:
Full catless/mufflerless stainless racing exhaust from the block back
3.5" Track grade racing intercooler (a denser cooler charge helps alot)
Car has been lowered about 2" inches (give or take)

I have also remapped the ecu and changed some of the AFR target settings. I can't really give a data-driven comparison for what removing the cat did, because there are other things being changed at the same time. I also refuse to change any of the hardware in my powertrain without retuning my ecu to optimize it, this goes for any vehicle I make any changes to. This is also why my Ranger has no powertrain mods of any sort.